1. Preheat the oven to 375 degrees F (190 degrees C).

2. Season boneless chicken with salt and pepper.

3. In a skillet over medium-high heat, add olive oil and cook chicken until browned on both sides, about 5 minutes per side. Remove from skillet and set aside.

4. In the same skillet, sauté onion and garlic until softened, about 5 minutes.

5. Add crushed tomatoes to the skillet and stir to combine. Season with salt and pepper.

6. Bring the tomato mixture to a simmer, then add the chicken back into the skillet. Spoon some of the sauce over the chicken and transfer the skillet to the preheated oven.

7. Bake for 20-25 minutes, or until the chicken is fully cooked through.

8. To assemble the sandwich, place one chicken breast on a hamburger bun and spoon some of the tomato sauce on top. Serve hot.