Butter 4 cups in a muffin tin. Cook bacon lightly on both sides, until almost done; drain, reserving 1 Tbsp for sautéing onion. Line the sides of each muffin cup with one strip of bacon. Sauté onion in bacon fat until translucent. Add kale to pan and cook to wilt. Season to taste with salt and pepper. Beat eggs, milk, cheese, and flour together and stir in sautéed onions and kale. Pour into the 4 lined cups. Bake at 350° for 30 minutes.