Season Chicken breasts with Salt, Black pepper, and Paprika.
Heat Olive oil in a large skillet over medium-high heat. Add Portobello mushrooms and Shallots and cook until softened and browned, about 6-8 minutes. Stir in Thyme during the last minute. Remove from skillet and set aside.
In the same skillet, melt Butter. Add Garlic and cook for 1 minute until fragrant.
Pour in Chicken broth and Dry Sherry. Bring to a simmer and cook for 7-10 minutes, reducing slightly.
Stir in Lemon juice and Dijon mustard.
Add Chicken breasts to the skillet and cook for 6-8 minutes per side, or until cooked through.
Top each Chicken breast with the cooked Mushroom mixture, shredded Monterey Jack cheese, and grated Parmesan cheese.
Cover the skillet and cook until the cheese is melted, about 2-3 minutes.
