Easiest. Recipe. Ever.
Onions, almost quartered but left intact at the core/root/whatever it’s called.
Salt and pepper (not called for in the cookbook, but I think an important addition)
Smear olive oil on the bottom and sides of a baking dish. Put onions in, intact side down. (Trim a bit to make them stand up.) Pour more olive oil over them. Season with salt and pepper. Cook at 500F for an hour. (You might want to open your kitchen window. It might get a bit smoky.)
Eat warm, or cold, or cut up in egg salad.