Set oven temperature to 375°F, or 190°C.
Slice the onions thinly, about 1/2 inch (1.27 cm) thick, after peeling them. Make a single layer in a baking dish after separating the slices into rings.
Blend the mayonnaise, shredded Parmesan, paprika, salt, pepper, garlic powder, and cheddar cheese in a small bowl. A delicious cheesy topping is made.
Cover the onion slices with the cheese mixture by spooning it on evenly.
Place the baking dish in the preheated oven and bake for approximately 25 minutes, covered with aluminum foil.
After ten to fifteen minutes after removing the foil, continue baking until the onions are soft and the top is golden and bubbling.
For a pop of color and a burst of flavor, top with chopped fresh parsley.
This delicious side dish goes well with a variety of main meals and is best served hot.