1. Heat 2 tablespoons of olive oil in a large nonstick skillet over high heat. Add shallots and garlic once oil is heated. Once translucent add the mushrooms and cook until lightly browned, about 7 minutes. Season to taste with salt and pepper. Set aside.
2. Heat the remaining 2 tbsps of olive oil in a large skillet over medium-high heat. Season the chicken with salt and pepper. Add to the skillet and cook, turning once, until the golden brown on both sides, about 8 minutes.
3. Place the chicken breasts in a baking pan and arrange mushrooms on top of breasts. In a small bowl, combine the mustard, herbs and salt and pepper to taste. Spread the mixture evenly over the chicken breasts and mushrooms. Bake until the breasts show no sign of pink when pierced in the center, 15 to 20 minutes.