This became an instant holiday classic the first time my mom added it to the Thanksgiving rotation. If you need to do anything in advance, sweat the onions. The final dish can get soupy if made in advance.
4 c sweet onions, sliced
1 c cream
2/3 c shredded parmesean cheese
Sweat onions in butter. Let cool slightly, add eggs and cream, combine, season with salt and pepper. Dump into 9x 9” or so casserole dish, top with cheese, bake 15 minutes at 425. The casserole should be quite thin, just over an inch thick. It may take longer to cook if it’s thicker.