How to Make It
Preheat oven to 400°. Fit piecrust into a 9-inch tart pan lightly coated with cooking spray. Bake crust 8 to 10 minutes or until lightly browned. Cool.
Heat butter and olive oil in a large skillet over medium heat until butter is melted; add onions and a pinch of salt. Cook, stirring often, 5 minutes. Cover and cook about 10 more minutes or until onions are soft and lightly browned. Uncover and cook, stirring often, 2 to 3 minutes or until liquid evaporates. Remove skillet from heat, and cool completely.
Whisk together crème fraîche and next 6 ingredients in a medium bowl. Stir in onions. Spread mixture in prepared crust.
Bake at 400° for 25 to 30 minutes or until golden and set. Cool on a wire rack 10 minutes before slicing.