Step 1

Preheat the oven to 220C/200C Fan/Gas 7.

Step 2

Heat the butter and oil together in a shallow, flameproof casserole over a low heat. Add the onion and cook for 5 minutes, stirring regularly until softened.

Step 3

Sprinkle over the flour and stir well then gradually add the milk, just a little at a time, stirring well between each addition to make a smooth sauce. Stir in the mustard and half of the cheese. Season to taste with salt and pepper.

Step 4

Stir in the gnocchi, broccoli, peas and tomatoes and bring to a gentle simmer, stirring.

Step 5

Sprinkle over the remaining cheese and bake in the oven for 12–15 minutes, or until golden-brown and bubbling. Carefully remove from the oven and serve.