Line a 10 to 12-inch tart pan with your favorite pastry shell. Lightly brush or spread the Dijon mustard on the shell. Sprinkle 1/2 cup Swiss cheese over the mustard. Peel the potatoes and cut them into large chunks. Boil potatoes in lightly salted water until tender. Drain the potatoes and then transfer to a large mixing bowl. Mash the potatoes. In a large saute pan melt the butter with the oil. Dice onions and saute in butter/oil mixture. Add the onion to the mashed potatoes. Add 1 cup of ricotta cheese, eggs and the remaining 1 cup of grated Swiss cheese to the potato/onion mixture. Add salt and pepper. Spoon mixture into the pastry shell. Bake for 45 minutes or until lightly brown on top.