Quiche is one of those dishes that need never be made the same way twice! The crust can be store-bought to make the process faster and the ingredients and proportions can be changed depending on what is on hand.
Spinach is a hardy green that can grow through the winter in more temperate climates, especially if a cold-frame is used. Chard, which can also grow through the winter (with a little protection), can be substituted for the spinach. Either green can be acquired during the summer growing season, blanched and frozen in appropriate amounts for use during the winter.
For another variation, sauté the sliced mushrooms in a little butter in a nonstick pan. Add fresh spinach or chard and cover skillet till spinach wilts. Remove spinach, chop it, and continue as above.
Another interesting egg dish that combines cheese, spinach and mushrooms is found in these variations of Hot Cheese Squares.