In the meantime, toast the bread to desired doneness in a toaster oven.* Layer 1 slice with the avocado, cooked mushrooms and onions, arugula, and finally the feta cheese, then top with the other slice.
Return the sandwich, feta side on the bottom, to the pan and press down gently for a few minutes, to make the cheese melt. Flip over, continuing to press down, for about 1 minute more.
Notes
*You can also do this in the pan after you've removed the mushrooms and onions, simply flipping it after a few minutes to toast the other side.
Recipe by Lenses and Lentils at https://lensesandlentils.com/mushroom-onion-feta-grilled-cheese/