Swap jarred salsa for the fresh version if you're making these out of tomato season. Slideshow: More Chicken Breast Recipes
In a medium saucepan, heat the oil over medium high heat until hot. Stir in the onion and cook, stirring until golden, about 6 minutes. Stir in the garlic and cumin and cook, stirring until golden, about 2 minutes. Stir in the tomatoes and cook until thickened, about 10 minutes. Stir in the lime juice and season to taste with salt and pepper. Let the salsa cool to warm.
Place each chicken breast, smooth-side-up, on a work surface. Place the palm of one hand on top of one breast, then insert a pairing knife to make a length-wise pocket in each breast. Divide some of the salsa between the breasts, stuffing them full. Rub the chicken with the oil, then season all over with 3/4 teaspoon salt and 1/2 teaspoon pepper. Spread the remaining salsa evenly over the breasts.
Preheat the oven to 425°F.
Place the chicken breasts on an oiled baking sheet and bake until cooked through, about 30 minutes. Let stand 5 minutes before slicing and serving.