Preheat the grill to high. Toss the mushrooms in a large bowl with the shallot, garlic and 2 tbsp of the oil.
Drizzle the bread with some olive oil, grill until lightly toasted, then set aside. Grill the mushrooms for 4-5 mins on each side until cooked. Divide the cheese between each of the mushroom cavities, sit the mushrooms on the toast, cheese side up, and put back under the grill and cook until the cheese is melting and bubbly.
Toss the spinach in the marinade bowl with the remaining oil and season if you want. Serve the spinach topped with the mushroom toasts.