Preheat the oven to 375 degrees F (190 degrees C).
Heat butter in a skillet over medium heat; cook and stir zucchini and onion until tender, about 10 minutes. Add salt, pepper, garlic powder, basil, and oregano; stir to coat.
Whisk eggs and mozzarella cheese together in a bowl. Stir zucchini mixture into egg mixture.
Separate crescent dough into two rectangles and place into a 8x12-inch baking dish; press into bottom and slightly up sides of dish to form a crust. Spread mustard onto crust. Pour zucchini-egg mixture into crust.
Bake in the preheated oven until set in the middle and lightly browned, 18 to 20 minutes. Cool before cutting into squares or wedges.