Here's one last dinner recipe to close out the week. This is from my book, Not Your Mother's Casseroles, and it really expresses the way I like to make vegetable casseroles. I find that many casseroles and gratins smother the vegetables with dairy, grains, and other peripheral ingredients. But I want to taste the vegetables — especially when, like here, they are fresh and springlike — leeks, fennel, potatoes, onion, garlic. So I let them stand on their own in this ultra-simple gratin. It's all about the vegetables.