Heat your oven to 180°C/Fan 160°C/Gas 4. Tip the quinoa into a sieve and rinse it under cold water for 1-2 mins, stirring it with your hand.
Tip the rinsed quinoa into a small pan. Toast for 1-2 mins to dry it out a little and start browning it. Pour in 300ml boiling water. Cover. Bring to the boil, then turn the heat down and simmer for 12-15 mins till all the water has been absorbed and the grains are tender.
While the quinoa simmers, slice the stalks off the mushrooms and finely chop them (set the tops aside). Pop them in a bowl. Peel and finely chop the garlic. Finely grate the zest from the lemon. Add them to the mushroom stalks.
Pick the leaves from the thyme sprigs. Add most of them to the mushroom stalks (keep a pinch back for garnishing).
When the quinoa is cooked, drain it (if necessary) and add half of it to the bowl. Add a pinch of salt and pepper and stir to mix. Taste the mixture and add more salt and pepper if you think it needs it. Set the remaining quinoa aside for later.
Put the mushroom tops in an ovenproof dish or small roasting tin. Spoon half the quinoa stuffing into the mushrooms. Crumble over a quarter of the pack of feta (approximately 50g). Pop the cherry tomatoes in alongside the mushrooms. Bake in the oven for 20 mins till the mushrooms are golden and the tomatoes are just bursting.
While the mushrooms bake, juice half the lemon. Whisk the juice with 1 tbsp balsamic vinegar, 2 tsp olive oil and a pinch of salt and pepper to make a dressing. Toss the salad leaves and remaining quinoa with the dressing. Crumble in another quarter pack of feta.
Serve the baked mushrooms with the salad and tomatoes. Slice the remaining lemon half into wedges for squeezing.