Wipe the portobello mushrooms clean and remove the stems. Finely chop or mince the garlic.
Lightly oil a baking dish or baking sheet and put the mushrooms on top in a single layer, open side up.
In a small bowl, mix together the oil, garlic, thyme, and a little salt and pepper. Divide the mixture between the mushrooms, drizzling over the inside. Top each with a thin slice of butter.
Bake the mushrooms for around 15 minutes until tender.
While they are cooking, mix together the cheeses. After the 15 min, top the mushrooms with the cheese and return to the oven for around 5 minutes until the cheese is melted and gently golden.
Notes
Note these mushrooms are great either as a side eg to steak/chicken or as a main with eg mashed potato or other veg. You could also serve them as an appetizer. Serving size around 1 mushroom per person as a side, 2-3 as a main.