Place oven rack in center of oven and heat oven to 350 degrees.
In a very large bowl, using a wire whisk, whisk eggs until frothy. Whisk in flour until incorporated; batter will look pasty. Whisk in milk, salt, pepper, baking powder and hot pepper sauce. Stir in cheeses and 10-12 ounces mustard greens, tossing to combine well. (Save rest of greens for another use.) It will look like there is too much mustard for the amount of batter, but dont worry.
Place butter in a 10-inch quiche dish, deep-dish pie pan or a 7- by 11-inch casserole dish. Place dish in oven until butter is melted, about 3 or 4 minutes. Remove hot dish from oven and tilt to swirl butter across bottom and up sides of pan a bit. Pour mustard mixture into heated dish (it will be piled high) and return to oven. Bake for 45-55 minutes or until puffed and golden brown. Remove from oven and cool 10 minutes. The frittata falls as it cools. Cut into wedges or squares and serve.
MAKE-AHEAD NOTE: This may be made a day ahead of time and kept in the refrigerator. Or the frittata may be baked, cooled and then frozen. To serve, defrost while covered, at room temperature and reheat, uncovered, at 350 degrees for 10-15 minutes or until heated through.
INGREDIENT NOTE: You may use pepper Jack cheese instead of regular Jack, and omit the hot pepper sauce if you like. Or use all cheddar if you prefer, or all Jack.