Preheat the oven to 425°F. Line a large rimmed baking sheet with parchment paper.
Spread the diced sweet potatoes out on the baking sheet. Drizzle with olive oil and sprinkle with 1/2 teaspoon kosher salt. Toss well, then roast for about 25 minutes, shaking the pan halfway through to give them a turn.
1 medium-large sweet potato, 2 teaspoons olive oil, kosher salt and black pepper
While the sweet potatoes roast, set a 10-inch cast-iron or other oven-safe skillet over medium-high heat. When hot, add the sausage and onion. Cook for 5-6 minutes, breaking up the sausage as it cooks.
8 ounces ground Italian or pork sausage, 1/4 cup chopped red onion
While browning the sausage, crack eggs into a large bowl and whisk well. Add the cheese, cream or half and half, thyme, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Whisk again to combine, then set aside.
8 large eggs, 1/3 cup shredded cheddar, 1/4 cup heavy cream or half and half, 1 teaspoon dried thyme
When the sausage is fully cooked, stir baby spinach and garlic into the skillet. Cook for about 1 minute, until the garlic is fragrant and the spinach is wilted. Remove from the heat and set aside.
2 cups fresh baby spinach, 2 garlic cloves
When the sweet potatoes are finished roasting, stir them into the skillet along with the sausage. Reduce the oven temperature to 350°F.
Pour the egg mixture over the sausage and veggies. Gently swirl the pan to distribute evenly.
Place skillet in the oven and bake for 18-20 minutes, until the eggs are set and pull away very slightly from the edges of the pan. The frittata may puff up in places; this is normal and will subside as it cools.
After removing the frittata from the oven, garnish with fresh thyme, parsley, or extra black pepper as desired. Slice into pieces, serve, and enjoy!
fresh thyme or parsley