Pre-heat the oven to 350 F Degrees. Butter each gratin dish lightly.
Heat olive oil in a large skillet. Sauté leeks until wilted, for 4-5 minutes, in medium heat. Stir in the garlic and cook for 1 minute.
Add kale and continue to sauté for 2 minutes until kale is softened.
Gradually whisk in the half and half, stirring constantly. Let it come to a boil. Let it simmer for 2-3 minutes or until it thickens. Season it salt, and pepper.
Take it off the heat.
Evenly divide sautéed leeks and kale mixture among 4 small gratin dishes. Sprinkle them evenly with Gouda cheese.
Using the back of a spoon, push filling to the sides creating enough space in the middle for the egg.
Crack eggs into individual bowls, taking care not to break yolks. Gently, pour eggs from cups into each gratin dish, centering yolk in the middle.
Place the gratin dishes in a deep baking tray and fill it with enough water to come half way up the size of the gratin dishes.
Carefully transfer them to the oven. Bake them for 15-20 minutes until the eggs are cooked to your liking. * After the first 15 minutes, keep a close eye on it as eggs can easily go from over easy to over hard.*
Season with salt and pepper. (I also like to season them with hot sauce, red pepper flakes and chopped parsley.)
Serve immediately with toast.