Preheat oven to 375 degrees and grease a 7x11-inch baking dish.
Peel the onions and cut into 1/3-inch slices. Melt half the butter in a large nonstick skillet over medium-low heat. Add onion slices in a single layer (you will have to cook them in batches) and cook for 3 to 4 minutes per side to soften them. Transfer to prepared baking dish. Season with salt and pepper.Repeat with remaining butter and onion slices.
In a liquid measuring cup, stir together heavy cream, sherry, black pepper and thyme.Pour over onions.
Top with swiss cheese, parmesan cheese, and panko crumbs.
Bake for 20 to 22 minutes or until light brown on top and bubbling.