Heat 1/2 Tbsp olive oil in a small pan.
Add chopped onion, sliced mushrooms, thyme, salt and pepper. Saute for 2 minutes.
Add white wine. Saute for another 3 minutes. Cover with a lid and remove from heat.
Separate egg whites from egg yolks.
Whisk egg whites in a large bowl. (you can use a hand mixer or a food processor)
Add salt and thyme. Mix.
Combine gluten-free flour with baking powder.
Add beaten egg yolks and combine together with the foamy egg whites using a spatula.
Start adding flour in the bowl, little by little, while continuously incorporating using a spatula.
When all flour is incorporated, add oil.
Mix easily until well blended together.
Grease a 30x10cm pan with some oil and powder it with flour.
Add batter and spread evenly.
Using a tablespoon, add the mushroom topping. Press it a little bit into the batter.
Bake in the preheated oven at 180C for 25min. Check with a toothpick if ready.