Cut peppers in half lengthwise and remove ribs and seeds. Place them on a parchment-lined baking sheet.: The moment you halve the Tribelli peppers you notice their bright color and sweet aroma, and removing the ribs and seeds creates a neat cup for the filling. Work carefully so the pepper halves remain intact, handling them gently to avoid tearing. A clean cut gives a tidy presentation and helps the filling sit evenly. One common mistake is leaving too many seeds inside which can add unwanted bitterness, so take your time scraping them out. Arrange the halves skin side down on the parchment-lined baking sheet so they sit level and the filling does not spill. Pressing lightly on the pepper base helps it stay steady while you fill later. If a half is too large or thick, you can trim the base slightly to make a stable cup, but avoid cutting away so much that the shell weakens.
Preheat the oven to 350 degrees.: As the oven warms to 350 degrees , the room gains a faint warm scent that hints at toasted edges to come. Preheating ensures even cooking, so the filling warms through at the same rate the pepper softens. If you skip preheating, you risk uneven texture where the filling overheats before the pepper softens, or the pepper becomes too dry. Use an oven thermometer if your oven runs hot or cool to maintain accurate heat. Place the rack in the center for balanced heat, and avoid opening the door often, which lets heat escape and lengthens bake time.
Make the filling by mixing the sausage, cream cheese, cheddar, and Parmesan together until well combined. Taste filling and add salt and/or pepper if needed.: When you mix the warm, crumbled mild Italian sausage with room temperature cream cheese , the filling takes on a silky texture, and the melting cheddar cheese and grated Parmesan cheese fold in to create depth. Stirring thoroughly ensures no pockets of cold cream cheese remain, which would affect the bake. Pause to taste; adjust with salt and freshly ground pepper so the filling sings when baked. A frequent error is over salting, especially if your sausage is already seasoned, so add in small increments and taste between additions. Aim for a mixture that holds together but still feels creamy, not runny. If the filling seems too loose, chilling briefly firms it and makes spooning easier.
Fill the peppers with the sausage mixture. Bake for 12-14 minutes. Serve warm.: Once filled, the visual cue you want is a slightly domed surface that browns gently at the edges during baking. As the pan slides into the oven you might hear a faint sizzle as trapped juices warm, and the aroma of melted cheddar cheese and roasted peppers will begin to rise. Baking for 12 to 14 minutes lets the filling heat through and the cheeses meld without drying the pepper. Avoid overbaking which can cause the filling to crack and the pepper to shrivel; if you see bubbling and a touch of golden color, they are ready. After removing from the oven, let them rest for a minute so the filling sets slightly, which makes them easier to pick up. A common pitfall is stuffing unevenly so some halves overfill and spill; spoon consistently and gently compress the filling so each piece bakes uniformly.