Heat a large skillet over medium heat. Add 1 teaspoon olive oil and crumble sausage into pan. Cook until sausage is no longer pink. Drain on paper towels.
In same pan over medium-low, heat remaining 2 teaspoons olive oil. Add onions, potatoes, and seasoned salt. Sauté, stirring occasionally, for about 5 minutes. Cover, reduce heat to low, and cook, stirring occasionally, for 5 more minutes or until potatoes can easily be pierced with a fork and onions are soft. (You want the potatoes soft, but not mushy.) Add spinach, black pepper, and cayenne pepper and stir for a minute or so until the spinach wilts. Stir in sausage and remove from heat.
Spray a 12-count muffin tin with cooking spray and preheat oven to 400°F.
Divide sausage mixture evenly among muffin cups. Let cool for 5 minutes or so, then sprinkle each cup with 1 tablespoon cheese.
In a medium bowl (ideally one with a pouring spout), whisk together eggs and yogurt. Divide egg mixture evenly among muffin cups.
Bake for 20 minutes or until puffy, set, and golden brown. Remove from oven and let cool for 10 minutes. Run a rubber spatula around each frittata and remove from pan. Serve immediately.