Sautee the mushrooms in olive oil in a large sautee pan, until moisture is released and mushrooms are cooked through. Add spinach, tearing up the leaves a bit as you add them to the pan (if you feel like you need another swig of olive oil added to the pan, go ahead and add it). Cook until spinach has wilted, then add feta to taste – we found adding a lot of feta was better. Sprinkle evenly with salt and pepper, to taste, and stir.
Use mushroom mixture as filling for the panini and cook paninis as you normally do, in a press or with two frying pans.
Recipe by This Week for Dinner at https://thisweekfordinner.com/spinach-mushroom-panini-with-feta/