Heat oven to 350 degrees. Thinly slice tomatoes. Lightly salt and pepper the tomatoes. Layer the tomatoes between paper towels for ten minutes.
Melt the butter in a skillet. Cook onions and shallots until translucent.
Mix mayonnaise and cheeses.
Lightly brush mustard in bottom of tart shells. Layer tomatoes and onions in shells. Sprinkle the tops of each tart with crumbled bacon. Top each tart with the mayonnaise cheese mixture. Place tarts on a baking sheet. Bake for 45 minutes.