Heat a pan or grill pan with oil. Saute garlic and onions until onions turn golden.
Add mushrooms and saute until they are cooked yet crunchy. Set these aside.
Smear ¼ to ½ tbsp butter over each bread slice.
Spread the mushrooms over the inner side of the bread (buttered side out).
Top the bread with mushrooms & grated cheese. Then sprinkle herbs and chilli flakes.
Place another slice of bread (buttered side out).
Toast this in a sandwich toaster or on a pan until the cheese melts. If doing it on a pan, toast it on a low heat and press down the mushroom sandwich with a spatula. Flip and gently toast again until the cheese melts.
Slice the mushroom sandwich and serve hot.