Heat olive oil and butter on medium heat in a thick-bottomed saute pan. Add the sliced onions, stir to coat and cook until golden brown, about 15-20 minutes. You can also do this step the day before and refrigerate until ready to assemble.

Preheat oven to 400 degrees.

In a large bowl, add the sliced potatoes and gently toss with 1 Tbls. olive oil and 1/2 cup of grated Gruyere cheese, 1/2 cup Parmesan, salt and pepper.

Butter a large oval gratin dish or 9 x 13-inch casserole lightly and line with the caramelized onions in an even layer at the bottom of the dish.

Arrange the sliced potatoes (including the grated cheeses) in a pattern over the onions. I did 4 rows with the potato slices standing upright and slanted back a bit.

Pour chicken stock or broth over the potatoes. Sprinkle with remaining 1/4 cup of Gruyere cheese.

Cover with foil and bake for 50 minutes (or until fork tender). Remove the foil. Broil for 5 minutes, uncovered, until lightly browned.

Sprinkle with rosemary if desired.

NOTE: I sliced the onions and potatoes with a mandolin. Warning: I now wear gloves when I use my mandolin after slicing my fingers once. Not a pretty sight!