Preheat your oven to 375 degrees Fahrenheit. In a large nonstick skillet, melt butter over medium-low heat then add in your shallot and cook until translucent, about 3 minutes.
Add in your diced zucchini and frozen mushrooms and cook until veggies are soft and liquid evaporates, about 5-7 minutes. If using fresh mushrooms, cook time will be on the shorter end given less liquid to evaporate.
Next, add in your chard and give the greens an occasional stir, allowing them to wilt over the next 3 to 5 minutes.
Prep your egg mixture during this last stage of cooking the vegetables by whisking together your eggs, egg whites, hot sauce, salt and pepper in a medium bowl until frothy.
Pour egg mixture into the skillet and allow the bottom of the frittata to set, about 1 to 2 minutes, before placing in the oven. Bake for 15-20 minutes until the center is set. Cool for 5 minutes before slicing and serving.