Crumble flour and butter in food processor. Combine egg yolk and milk, add and mix until dough comes together. Roll out to fit into a 9” tart pan. Freeze for 15 minutes. Line crust with foil and weights and bake at 425 degrees for 15 minutes.
Saute onions in butter and oil for about 10 minutes until soft. Add salt and wine and cook until about ¼ cup liquid remains. Add eggs, cream and half of cheese to onions, mix and put into cooled tart shell. Top with remaining cheese and bake at 400 degrees for 30-40 minutes until golden brown.