Add 2/3 cup dry quinoa and 1 1/3 cup water or stock to a small saucepan with a lid. Season with a generous pinch of salt. Bring the mixture to a boil (uncovered) over medium-high heat. Reduce heat to low or medium-low, bringing the quinoa to a slow simmer. Cover and simmer for 15 minutes. Remove from the heat & let stand (covered) for 5 minutes. Uncover & fluff with a fork. Set aside.
Add the olive oil to a skillet over medium heat. Add in the onion, pepper, chili flakes, & a pinch or two of salt. Sauté, stirring occasionally, for about 5 minutes, until softened & translucent. Add sliced chicken sausage & sliced baby portobello mushrooms to the skillet, stirring to combine. Cook for 5-7 minutes until nicely browned, stirring occasionally to avoid burning.
Once the sausage & mushrooms are nicely browned, stir the shredded kale, quinoa, & tomato sauce into the sillet. Cook until the kale begins to wilt & the sauce begins to bubble, about 3 minutes. If using, place the cheese over top the skillet & allow it to melt, about 2 minutes longer.