1. Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). In a large frying pan, sauté onion and leek in olive oil for 10 minutes over medium-low heat. Add cauliflower and continue to sauté an additional 10 minutes, stirring and separating pieces, until cauliflower is soft.
2. Remove from heat and stir in the eggs, onion soup mix, and mayonnaise. Mash together. Pour mixture into frozen pie shell. (You will have a little extra mixture to make an additional small tart or six to eight mini tarts.)
3. Bake for one hour. Mini tarts will only need a half hour.