Wipe the mushrooms with a clean, damp kitchen paper and cut out the stems. Mix the olive oil and garlic together in a bowl, then brush the garlic infused oil over the underside of the mushroom caps. Lay the mushrooms gill-side up on a baking sheet and lightly score with the tip of a sharp knife, taking care not to cut all the way through the caps.
Drizzle the maple syrup and vinegar over the gills, then sprinkle with thyme and season with salt and freshly ground black pepper. Lay the oily garlic slices over the surface and finish with the remaining oil. Roast in the oven for 15-20 minutes, or until soft, juicy and sizzling.