Preheat the oven to 200 degrees. Halve the mushrooms and place on a tray. Flatten the garlic cloves with the back of a knife to bruise slightly and place on the other tray. Cut the fennel bulbs in half and add to the tray. Drizzle both trays of vegetables with the olive oil. Roast the vegetables for 25-30 minutes until golden at the edges. Leave the vegetables to cool slightly, then blitz in a blender with the stock and thyme leaves. Taste for seasoning. This soup can take plenty of salt and pepper. Reheat and serve with a dollop of creme fraiche and fresh thyme leaves.