Heat your oven to 200°C/Fan 180°C/Gas 6. Trim the roots and top 3cm off the leeks. Halve them lengthways, rinse out any grit and finely chop them. Peel the garlic and crush or finely chop it. Finely chop the rosemary leaves. Carefully pull the stalks off the portobello mushrooms. Finely chop the stalks, setting the mushroom caps aside.

Pour 1 tsp olive oil into a large frying pan and warm to a medium heat. Slide in the leeks, mushroom stalks, garlic and rosemary. Season with a little salt and pepper. Fry for 10 mins, stirring occasionally, till softened.

Meanwhile, scrub the potatoes and cut them into bite-size chunks. Tip the chopped potatoes into a pan and cover with cold water. Add a pinch of salt and pop on a lid. Bring to the boil, then simmer for 10 mins, till the potatoes are tender.

When the leeks are cooked, stir through 1 tbsp wholegrain mustard and the baby leaf spinach. Pop the portobello mushroom caps in a roasting tin, gill-side up. Add plenty of leek and mustard filling to each mushroom. Scatter the breadcrumbs over the top. Slide the mushrooms into the oven to bake for 20 mins, till golden.

While the mushrooms bake, peel the red onion and slice it as thinly as possible. Pop it in a bowl. Grate over the lemon zest and squeeze in the juice. Add a small pinch of salt. Scrunch together with your hands and set aside.

Check the potatoes – they should be tender and cooked through. Drain them and leave to steam dry in the colander for 2 mins. Tip the potatoes into a large bowl. Pour over the red onion and lemony pickling liquid. Pour over 1 tbsp olive oil and season with black pepper. Stir to coat.

Fold the rocket into the potatoes. Divide the stuffed mushrooms between a couple of plates and serve with the potatoes on the side.