Heat 1 tbs olive oil in a frypan over medium high heat. Add the mushrooms, season with salt and pepper, and cook, tossing for 3-4 minutes until golden. Remove from heat and cool slightly and place in a bowl.
Slice the bread and toast.
Meanwhile, split avocado in half lengthways. Brush the half with the stone with lemon juice, wrap in cling film and refrigerate. Chop the remaining avocado, place in the bowl with the mushrooms and drizzle over lemon juice to stop it from going brown.
Add the feta, season and toss to combine. Drizzle toast with remaining 1 tbs olive oil and place on top of sourdough toast.