Preheat oven to 350. Spread the mustard on the bottom and sides of pie crust.
Saute the onion in butter until soft. Add zucchini slices and saute until softened. Drain the onion/zucchini mixture and add to pie crust. NOTE: Draining the onion/zucchini mixture is very important as this keeps the bottom of the pie crust from getting too soft.
In a bowl, slightly beat the eggs then add the cheese and seasonings. Pour over the onion and zucchini. Bake for 20-30 minutes or until the top is bubbly and lightly browned.