For perfectly tender peppers, ensure all internal membranes and seeds are meticulously removed, as they can impart a bitter flavor. Initially covering the baking dish during cooking steams the peppers, leading to a wonderfully soft texture, while uncovering allows the cheese to achieve a beautiful golden crust. Properly browning the ground beef before adding it back to the skillet is crucial; ensure all excess fat is drained to prevent a greasy final product. Finely dicing the onions, zucchini, and reserved pepper tops ensures they cook down evenly and integrate seamlessly into the stuffing, providing a harmonious texture. Consider cooking the rice in a good quality broth for an extra layer of savory depth.