• 50g breadcrumbs
• 20g butter
• For the crumb topping:
• 150g Gruyu00e8re, grated
• u00bd tbsp Dijon mustard
• 120ml cream, at room temperature
• Salt and black pepper
• 250ml milk, at room temperature
• 60g plain flour
• 60g butter
• 2 onions, sliced
• 2tsp olive oil
• 220g Brussels sprouts, quartered
• 350g cauliflower, chopped into florets
• 300g broccoli, chopped into florets

1. Preheat the oven to 180˚C/160˚C fan/gas mark 4.
2. Bring a large pot of water to the boil. Add the broccoli, cauliflower and Brussels sprouts and cook for 2-3 minutes until the broccoli turns vibrantly green.
3. Transfer the vegetables to a large bowl of ice water for two minutes, then drain and place in a large casserole dish.
4. Heat the olive oil in a saucepan over a medium-high heat and cook the onions for 4-5 minutes until golden and soft, stirring occasionally.
5. Reduce the heat to medium-low heat and add the butter. Once melted, gradually mix in the flour, whisking constantly. Continue cooking and whisking for 1-2 minutes.
6. Gradually add the cream and milk, whisking constantly. Simmer for 4-5 minutes until the sauce thickens.
7. Remove the saucepan from the heat and stir in the mustard, half of the Gruyère and some salt and black pepper to taste.
8. Pour the sauce over the vegetables, then top with the remaining cheese.
9. Melt the butter for the topping in a bowl and stir in the breadcrumbs until combined, then sprinkle over the casserole.
10. Bake for 20-25 minutes until golden on top and bubbling around the edges.