1 (10 to 12-inch) pastry tart shell, uncooked
3 tablespoons Dijon mustard
1 1/2 cups grated Swiss cheese, divided
4 to 5 baking potatoes, peeled and cut into large chunks
2 tablespoons butter
2 teaspoons olive oil
1 large Spanish onion, diced (2 cups)
1 cup ricotta cheese
3 large eggs
Salt and pepper, to taste