Step 1: Preheat your oven to 375°F (190°C).
Step 2: In a 10-inch oven-proof skillet (preferably cast iron), heat the olive oil over medium heat. Add the diced sweet potatoes and red onion.
Step 3: Sauté the potatoes for 8-10 minutes, stirring occasionally, until they are golden brown and tender when pierced with a fork.
Step 4: Add the chopped kale and garlic to the skillet. Sauté for another 2-3 minutes until the kale has wilted and the garlic is fragrant, ensuring everything is well-distributed for the sweet potato and kale frittata.
Step 5: In a medium bowl, whisk together the eggs, milk, salt, and pepper until well combined.
Step 6: Pour the egg mixture over the vegetables in the skillet. Use a spatula to ensure the vegetables are submerged and evenly spread. Sprinkle the crumbled goat cheese over the top of the sweet potato and kale frittata.
Step 7: Let the frittata cook on the stovetop for 2 minutes without stirring to allow the bottom to set.
Step 8: Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the eggs are completely set in the center and the top is lightly golden.
Step 9: Remove from the oven and let rest for 5 minutes before slicing and serving your sweet potato and kale frittata.