• In a large frying pan, heat 1 tablespoon butter and 1 tablespoon oil over medium heat. Add onions and stir to coat with the butter and oil mixture. Spread the onions out evenly over the pan and let onions cook slowly, stirring occasionally, for 10 minutes. Turn heat up to medium high and begin to brown the onions, stirring constantly about 7 more minutes. Season with salt and pepper. Transfer caramelized onions onto a plate and set aside.
• In another pan, heat the remaining butter and oil over medium-high heat. Add the mushrooms and sauté for 5 to 7 minutes or until liquid has evaporated and mushrooms are tender and lightly browned. Add spinach and stir until lightly wilted. Season with salt and pepper to taste.

Assemble the sandwich:

• Toast the slices of bread. Layer bottom slice with caramelized onion, followed by the sautéed mushroom mixture, a sprinkle of crumbled feta cheese and top with the other slice of bread.