Lightly toast the bread on both sides under a grill (broiler).Heat the butter and oil in a small frying pan/skillet. Place the mushrooms in the pan and fry on a medium-high heat for 5-6 minutes until the mushroom turn brown and a little bit crisp at the edges. Then turn down the heat and stir in the salt, pepper and garlic. Cook for 1 more minute, then turn the heat down to very low.Place the slices of brie on the toast and put back under the grill to melt. Once melted, place on two plates.Strip two of the thyme sprigs of the little leaves and stir through the garlic mushrooms. Spoon on top of the brie, then top each toast slice with a sprig of thyme and a grind of black pepper.