Heat a skillet over medium heat and add 1 tablespoon of olive oil. Add shallots and peppers with salt and pepper, cooking until soft – about 5 minutes. Add in garlic and spinach, cooking until spinach wilts, then stir in smoked paprika and parmesan.
Remove spinach mixture from skillet and set aside in a bowl. Add remaining olive oil in skillet, then place mushrooms in the pan stem side down. Cook for 5-6 minutes, then flip. Cook for 5 minutes more, then fill each mushroom with an equal amount of the spinach mixture. Turn the heat under the skillet off, and heat the broiler in your oven.
Dizzle balsamic into pan and gently stir mushrooms to coat, then add a slice of fontina on top of each. Place under the broiler for 2-3 minutes, or until cheese gets bubbly and melts. Serve immediately, garnished with parsley if desired and with crusty bread.