Preheat your oven to 375°F and position a rack in the center position.
Heat 1 tablespoon olive oil in a 10-inch oven-safe skillet over medium heat, then add the diced sweet potato and cook for 8-10 minutes until slightly softened, stirring occasionally.
Add the chopped onion and minced garlic to the skillet and cook for 3-4 minutes until the onion becomes translucent and fragrant.
Whisk together 8 eggs, 1/4 cup milk, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a medium bowl until well combined and slightly frothy.
Pour the egg mixture evenly over the vegetables in the skillet, then sprinkle 1/2 cup shredded cheddar cheese on top without stirring.
Transfer the skillet to the preheated oven and bake for 18-20 minutes until the eggs are fully set and the top is golden brown.
Remove the frittata from the oven and let it rest in the skillet for 5 minutes to finish setting up before slicing.
Garnish with 1/4 cup fresh chopped parsley, then slice into wedges and serve warm.