Preheat the oven to 200C/400F degrees.
Heat the butter in a pan on a medium heat.
Add the garlic and cook for 2-3 minutes.
Finely chop the mushrooms and add to the pan.
Cook for 8-10 minutes until the mushrooms are cooked.
Stir in the thyme and season with the salt and pepper. Remove from the heat.
Place the mushroom mixture in a bowl and add the egg yolks and blend (I use a hand blender), so the mixture is smooth.
In another bowl, whisk the egg whites.
Gently fold the egg whites into the mushroom mixture.
Grease and line a rectangle baking tray with parchment paper.
Spread the mixture evenly in the baking tray and bake for 15 minutes until firm and slightly golden.