Turn on the oven to preheat to 400 degrees. Prepare the onion rings by peeling a large onion and slicing the ends off. Cut the onion through the middle and separate the largest rings from each half, with each ring being about 2 layers thick. Each serving will need 1 double-layered ring about 1 -¼ oz in weight. Rub ¼ teaspoon of olive oil all over each onion ring and season the rings with salt and pepper. Arrange the rings on a well-seasoned sheet tray with the wider parts of the rings facing up.
In a mixing bowl, combine ground Italian sausage with mayonnaise and garlic powder. You can use either ground Italian sausage that is packaged or you can remove the casing from links of sausage. Simply use a clean hand to mix the meat, mayonnaise, and seasoning until you have a consistent mixture. Gently press the Italian sausage mixture into the prepared onion rings until each serving is filled to the top of the rings.
Lay a slice of cheese over every portion. Bake the tray in your oven for approximately 25 minutes. The onions should be soft and at least partially translucent. They’ll be very soft but fully intact. Look for excess fat that has released from the ground sausage. You can also test the center of the sausage portions with a meat thermometer, looking for at least 160 degrees internally. The cheese will have a browned, bubbly top. At your discretion, you can cook the stuffed onion rings under a broiler for additional time for more browning.