Place the mushrooms, stalk sides up, in a large roasting tin and season with salt and pepper. Put the garlic, oil and thyme in a bowl. Add the lemon rind, reserving some for a garnish, then mix together. Spoon half of the sauce over the mushrooms.
Roast the mushrooms in a preheated oven, 220°C (425°F), Gas Mark 7, for 20 minutes, until tender. Sprinkle with the parsley and drizzle over the lemon juice.
Arrange the mushrooms on the buttered toast, drizzle over the remaining oil mixture and serve topped with some rocket, the remaining lemon rind and some Parmesan.