Preheat the oven to 400°F (200°C). With a sharp knife, slit each chicken breast in half horizontally and fold open like a book; do not sever the halves completely. Season with salt and pepper.
In a food processor or by hand, chop together the garlic, sage leaves, 1 1/2 ounces pancetta, butter, and bread crumbs. Lay the chicken breasts open on a work surface and divide the filling among them. Fold over the other side to close and press gently to seal.
Oil a small baking or gratin dish and arrange the stuffed chicken breasts in the dish in a single layer. Season with salt and pepper and lay a slice of pancetta over each one.
Bake for 30 minutes. Raise the oven temperature to 425°F (220°C) and bake for 10 minutes. Transfer to a preheated broiler and grill about 6 inches from the heat until lightly browned, 10 more minutes.