If you cook onions slowly over low heat, they lose their sharpness and turn melty soft and sweet. That caramelization is what makes this French beauty a classic. I followed a recipe by André Soltner of Lutèce fame that can be found in NYT Cooking.  The caramelized onions are combined with cream, eggs, a sprinkle of salt, pepper, and nutmeg, then baked in a savory tart crust. The result is pure buttery, flaky, onion magic.