Preheat your oven to 375 F.
In a bowl, combine the tomatoes, mushrooms, onion, garlic, balsamic vinegar, and olive oil, and season with salt and pepper to taste.
6 cremini mushrooms, 3 Roma tomatoes, ½ onion, 2 garlic cloves, 1 tbsp. balsamic vinegar, 2 tbsp. olive oil, Sea salt and freshly ground black pepper, 2 tablespoon fresh basil
Place the chicken breast on a work surface. Slice through the breast on the long side (with your knife parallel to the countertop) almost to the end, but don’t cut all the way through.
4 boneless
Unfold the chicken (it should open like a book), and flatten using a meat mallet.
Divide the mushroom-tomato mixture evenly among the chicken breasts. Spread it on top of each breast, leaving 1 inch of space free around the edge.
Roll up each chicken breast, making sure to keep the mixture inside. Secure with cooking twine or toothpick.
Cooking twine or toothpicks
Melt some cooking fat in an oven-proof skillet over high heat and brown the chicken breasts on all sides, 2 to 3 minutes per side.
Cooking fat
Place in the oven and bake for 25 to 30 minutes or until chicken is cooked through.
Let the stuffed chicken breasts rest for 3 to 4 minutes before slicing and serving.