Autumn roasted squash, sweet chestnut & chestnut mushroom gratin
Warming autumn gratin of roasted butternut squash, sweet chestnuts and chestnut mushrooms in a creamy mustard sauce, topped with sage breadcrumbs.
Conker season signals crunchy leaves, scudding skies and a craving for something cosy. This gratin swaps the usual potato for roasted butternut and celebrates autumn with sweet chestnuts and earthy chestnut mushrooms. Creamy, mustard-laced sauce and a crisp sage-breadcrumb topping make it proper comfort food — great for a midweek supper or a weekend sharing dish with a green salad and pickles.
Ingredients
- 900 g butternut squash
- 50 ml olive oil
- 5 ml fine sea salt
- 5 ml black pepper
- 300 g chestnut mushrooms
- 200 g peeled sweet chestnuts — vacuum-packed or tinned peeled chestnuts
- 1 brown onion — medium
- 3 garlic cloves — crushed
- 8 fresh sage leaves
- 25 g plain flour
- 400 ml vegetable stock — hot
- 150 ml double cream
- 120 g vegetarian mature cheddar — grated
- 5 ml Dijon mustard
- 1 lemon — zest only
- 50 g fresh breadcrumbs
- 25 g unsalted butter — plus a little extra for greasing
Equipment
- baking tray
- ovenproof gratin dish (approx. 20 x 30 cm)
- large frying pan
- box grater
Instructions
Preheat the oven to 200°C fan/220°C conventional. Lightly grease a baking tray and an ovenproof gratin dish (approx. 20 x 30 cm).
Peel, halve and scoop out the seeds from the butternut squash. Cut into even 2–3 cm cubes, toss with 25 ml olive oil, half the fine sea salt and half the black pepper, then spread in a single layer on the baking tray. Roast on the middle shelf until tender and caramelised at the edges, 35 minutes.
While the squash roasts, thinly slice the chestnut mushrooms, finely chop the brown onion, crush the garlic cloves and roughly chop the peeled sweet chestnuts. Heat the remaining 25 ml olive oil in a large frying pan over a medium hob and cook the brown onion for about 6 minutes until soft. Add the chestnut mushrooms, garlic cloves, chopped peeled sweet chestnuts and three-quarters of the fresh sage leaves; season with the remaining fine sea salt and black pepper and cook until the mushrooms have released their liquid and started to brown, 8 minutes.
Sprinkle the plain flour over the mushroom mixture, stir thoroughly and cook for 1 minute to remove the raw flour taste.
Gradually pour in the hot vegetable stock, whisking or stirring constantly to avoid lumps. Bring to a gentle simmer and cook until the sauce has thickened slightly, about 4 minutes.
Stir in the double cream, Dijon mustard and the zest from the lemon. Check seasoning and simmer for 2 minutes to marry the flavours.
Fold the roasted butternut squash into the mushroom–chestnut sauce so everything is coated. Spoon the mixture into the prepared ovenproof dish and scatter the grated vegetarian mature cheddar evenly over the top.
Melt the unsalted butter in a small pan, stir in the fresh breadcrumbs and the remaining chopped fresh sage leaves until evenly moistened. Scatter the sage breadcrumbs over the cheese layer. Bake on the middle shelf at 180°C fan/200°C conventional until the topping is crisp and golden and the filling is bubbling, 25 minutes.
Leave the gratin to rest for 5 minutes before serving so the sauce settles.
Tips
- Cut the squash into uniform 2–3 cm cubes so they roast evenly — smaller pieces will char, larger pieces will remain firm.
- If you can't get vacuum-packed peeled chestnuts, use tinned peeled sweet chestnuts; add them straight to the pan with the mushrooms.
- Prepare the breadcrumb topping ahead and refrigerate; grate the cheese and zest the lemon before you start to speed things up.
- If the topping is browning too fast under the grill, tent with foil and finish until the filling bubbles through.
- Leftovers keep in the fridge for 3 days; reheat covered at 160°C fan/180°C conventional until piping hot.
Substitutions
butternut squash → 500 g pumpkin or 900 g crown prince squash, same preparation
Stays closepeeled sweet chestnuts → tinned peeled chestnuts, drained (use same weight)
Stays closevegetarian mature cheddar → gruyère or comté (120 g), will give a nuttier, more elastic top
Noticeably differentfresh breadcrumbs → 50 g panko breadcrumbs for a lighter, crunchier topping
Stays close
Nutrition
Per serving · estimated
- 750
- Calories
- 19 g
- Protein
- 67 g
- Carbs
- 45 g
- Fat
- 9 g
- Fibre
- 10 g
- Sugar
- 650 mg
- Sodium
FAQ
Can I make this dairy-free?
Yes — swap double cream for a full-fat oat cream and replace the vegetarian cheddar with a dairy-free melting alternative. The texture will be slightly looser.
Can I use whole chestnuts and roast them myself?
You can, but roasting and peeling sweet chestnuts takes 30–40 minutes extra. If you do, score and roast them first, then roughly chop before adding to the sauce.
Can I assemble ahead and bake later?
Yes — assemble up to the point of adding the breadcrumb topping, cover and refrigerate for up to 24 hours. Add topping and bake from chilled, adding an extra 5–10 minutes.
Is this suitable for vegetarians?
Yes — the recipe uses vegetarian mature cheddar so it is suitable for vegetarians as written.
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