Crack the eggs into a large bowl and season with salt and pepper. Whisk until well blended. Set aside.
Heat a 12-inch, nonstick skillet over medium heat and add the olive oil. Add the smoked sausage and cook, stirring frequently, until lightly browned, 5-6 minutes
Add the mushrooms and scallions and cook until the mushrooms soften and release their juices, 3-4 minutes. Add the chopped kale and season with salt and pepper. Cook until wilted, about 3 minutes.
Increase the heat to medium-high and pour the whisked eggs over the skillet ingredients. Tilt the skillet gently in a circular motion to allow the eggs to cover the ingredients in an even layer. (Some ingredients will still be visible above the eggs.)
Cook until the bottom and sides of the frittata are nearly set and the top is still runny but beginning to set along the edges, about 3 minutes.
Scatter the cheese on top and move the skillet to the broiler. Broil until the top is set and begins to brown, 2-3 minutes. Remove and let rest 5 minutes.
Scatter chopped parsley on top and cut into 4 wedges for serving.
Frittatas are easy, one-skillet meals that come together quickly with only few ingredients. Here, smoked sausage brings meaty flavor, but only half a pound total equals just two ounces of sausage per serving. Mushrooms bring their own meatiness to the dish, and earthy kale adds “super-food” goodness.