Preheat your oven to 200°C/Fan 180°C/Gas 6. Pop the dried porcini mushrooms into a heatproof measuring jug and cover with boiling water. Leave for 5 mins to soak, then drain and rinse well under a cold tap.

While the porcini soak, trim the roots and top 3cm off the leeks. Rinse well under a cold tap, then slice into thin rounds. Scrub the sweet potatoes and cut them into small 1-2cm chunks (no need to peel unless you prefer to). Thickly slice the Portobello mushrooms. Peel and finely chop the garlic.

Pour 1 tbsp oil into a large frying pan and warm to a medium heat. Slide in the chopped leeks, sweet potatoes, mushrooms and garlic. Season with a pinch of salt and pepper. Fry for 10 mins, stirring occasionally, till the veg have softened slightly.

Stir the flour into the veg and cook for 1-2 mins, stirring to stop it from catching on the bottom of the pan. Slowly pour the milk into the pan, stirring frequently, till the mixture becomes a smooth sauce. Add the porcini mushrooms and bring the pan to a simmer.

Add the baby leaf spinach to the pan along with 1 tbsp wholegrain mustard. Grate in the cheddar. Stir till the spinach has wilted and the cheese has melted. Have a taste of the sauce and add more salt or pepper if needed. Pour the veg and sauce into an ovenproof dish. Scatter over the breadcrumbs. Slide the dish into the oven and bake for 20 mins, till golden and bubbling on top.

When the bake is nearly ready, tip the rocket into a bowl and juice the lemon over the leaves. Toss to dress the rocket in the lemon juice.

Divide the cheesy mushroom bake between a couple of plates, and serve with the lemony rocket salad on the side.