Heat your oven to 200°C/Fan 180°C/
Scrub the potatoes and chop them into small pieces. Tumble into a pan and cover with boiling water. Sprinkle in a little salt, pop on the lid and bring to the boil. Once it's boiling, turn the heat down a little and simmer for 15 mins till tender.
Meanwhile, carefully pull the stems away from the portobello mushrooms. Lay the mushroom caps in a roasting tin. Finely chop the mushroom stems and slide into a bowl. Crumble in the Perl Las cheese. Peel and grate in 1 of the garlic cloves and scatter in the thyme leaves. Sprinkle in a small pinch of salt and pepper and stir to combine.
Spoon the cheese mixture into the mushrooms. Sprinkle over the breadcrumbs. Peel the red onions and cut them into wedges. Arrange the wedges in the gaps between the mushrooms in the roasting tin. Drizzle over 1 tbsp oil and roast for 20 mins.
Peel and crush or finely grate the remaining garlic clove. Roughly shred the spinach leaves.
When the potatoes are cooked, drain them well and return to the pan. Use a fork or wooden spoon to roughly crush the potatoes. Add the garlic and spinach, along with 1 tbsp oil and a good pinch of salt and pepper. Set the pan over a medium heat and stir for 2 mins till the spinach has wilted.
Spoon the potatoes onto plates and pop a couple of the stuffed mushrooms and onion wedges on the side. Finish with a scattering of peppery salad leaves and serve.