Prepare the Egg Mixture : In a large mixing bowl, whisk together the eggs, milk, salt, black pepper, dried rosemary, and dried thyme until well combined. Stir in the shredded Manchego or Iberico cheese and set aside.

Cook the Sausage : In a large oven-safe skillet, heat 1 tablespoon of olive oil over medium heat. Add the sweet Italian sausage, breaking it up into small pieces with a spatula. Cook until the sausage is browned and cooked through, which should take about 5 to 7 minutes. Once done, remove the sausage from the skillet and set it aside, leaving any rendered fat in the pan.

Sauté the Vegetables : Add the remaining olive oil to the skillet, then add the minced onion and diced sweet pepper. Sauté for 3-4 minutes, or until the onions are translucent and the peppers begin to soften. Add the shredded sweet potato to the skillet and cook for an additional 3-4 minutes, stirring occasionally, until the sweet potato softens slightly.

Combine and Pour the Frittata Mixture : Return the cooked sausage to the skillet, spreading it evenly among the vegetables. Pour the egg mixture over the sausage and vegetables, gently stirring to ensure an even distribution.

Cook on the Stovetop : Reduce the heat to low and cook the frittata for about 5 minutes, or until the edges start to set.

Bake in the Oven : Preheat your oven to 350°F (175°C). Transfer the skillet to the oven and bake for 15-20 minutes, or until the frittata is fully set and the top is lightly golden.

Serve : Carefully remove the skillet from the oven and let the frittata cool for a few minutes before slicing. Serve warm and garnish with fresh herbs if desired.