In a large pot, melt together 2 tablespoons butter and olive oil over medium heat. Add the sage and cook for about a minute to crisp. Mix in the onions and salt. Cook for a minute, then reduce heat to medium low. Cover and cook, stirring occasionally, until golden and caramelized, 30-45 minutes. Transfer the onions to a bowl and allow to cool to room temperature.
Preheat oven to 425˚F. Mix the eggs and heavy cream into the cooled onions. Divide among the partially baked pie dishes. Top evenly with cheese.
Arrange a few apple slices flat over the filling around the edge with the peel side facing the crust. This will create a base for layering the remaining apple slices.
Starting along the edge, layer the apple slices in a circular pattern, alternating the direction if desired and if room is available using the larger tart pan (there wasn't enough space in the 4 inch tart pans). Brush the tops with melted butter and bake in the 425˚F oven until the filling is set and the crust is golden, about 30 minutes (closer to 40-45 minutes for a 9 inch tart). If the edges of the apples are starting to burn, cover lightly with foil and reduce heat to 375˚F.
Serve warm or at room temperature.