Preheat oven (electric cooker: 220 °C/circulating air: 200 °C/gas: see manufacturer). Wash the thyme, shake dry, remove leaves from the stalks. Peel garlic and chop finely. Wash the lemon hot, dry it off, rub the peel thinly. Clean the mushrooms, possibly wash and halve them.
Roast bread slices on a baking tray in a hot oven for about 5 minutes. Heat the oil in a large pan and fry the mushrooms for about 4 minutes. Season with garlic, lemon, thyme, salt and pepper. Cover slices of bread with cheese and melt briefly in a hot oven. Arrange bread with mushrooms.