Preheat the oven to 450 degrees F (230 degrees C).
Spray a medium skillet with olive oil spray, then add the onion and mushrooms and saute until the onions are slightly translucent and the mushrooms are softened. Set aside.
½ cup chopped onion, 8 ounces of fresh mushrooms
Butter one side of each of the 8 slices of bread with the butter.
buttery spread or butter, 8 slices of Italian bread
Place 4 of the slices on a baking sheet, buttered side down.
Spread a generous tablespoon of pesto on each of these 4 slices of bread, then layer with a slice of mild cheddar, then ¼ of the mushroom mixture, then top with 1 slice of mozzarella.
½ cup Broccoli Pesto, 4 slices mild cheddar cheese, 4 slices mozzarella cheese
On the remaining 4 slices of bread, spread the non-buttered side with a tablespoon of pesto, then place them each on top of the mozzarella, buttered side up.
Bake in the oven for 6-8 minutes, then flip the sandwiches and bake about 6 minutes more, until the sandwiches are golden brown and crispy. (Alternatively, you can grill them on the stove top on a griddle or in a skillet).