Use a fork or a whisk to death the eggs and egg whites in a small bowl until creamy and thick. Set aside.
Squeeze the chopped spinach by handfuls over the sink to remove excess moisture. Set aside.
Heat the olive oil in a 10-inch, nonstick skillet over medium heat. Add the mushrooms; cook, stirring often, until they give off some of their liquid and it then evaporates, about 5 minutes.
Crumble in the spinach; stir in the oregano, salt, pepper and red pepper flakes. Stir for 2 minutes to warm the spinach and toast the spices.
Beat the eggs one more time to make sure they’re cream, then pour them into the skillet. Reduce the heat to low, cover and cook until set and no longer loose, about 10 minutes.
Loosen the frittata with a heat-safe rubber spatula and slide onto a cutting board to slice into quarters. Serve and enjoy!