Preheat oven to 375°F (191°C).

In a medium bowl, preferably one with a pour spout, whisk together the eggs, egg whites, milk, salt, and pepper. Whisk in the cheese. Set aside.

Heat olive oil in a 2-inch-deep, 10- to 12-inch-diameter oven-safe skillet over medium heat. Add the onion, bell pepper, and garlic. Cook for 5 minutes, stirring occasionally, until the onions and peppers begin to soften. Add sliced chicken sausage and cook for 3 minutes, stirring occasionally.

Pour the egg-and-cheese mixture over the vegetables and sausage and cook for 5 minutes, without stirring, until bubbling/starting to set around the edges.

Carefully transfer the skillet to the center rack of the oven, and bake for 22–26 minutes, or until the frittata is puffy and the center no longer wobbles/looks wet.

Transfer the skillet to a wire rack, and let it cool for at least 10 minutes before slicing and serving. Frittata will continue to set as it cools. Top with fresh arugula for serving, if desired.