Frittatas are one of the most efficient vehicles for leftover vegetables. Here, we pair roasted butternut squash with quick-cooking kale for a fiber-rich breakfast duo. A touch of dairy lends creaminess to the egg mixture, which helps achieve the coveted custardy frittata texture. For best results, use a well-seasoned cast-iron skillet and pull it from the oven before the eggs are completely set. Because it retains heat well, a cast-iron will continue to cook your frittata after you remove it from the oven.