Potato Gratin

2 Tablespoons olive oil
2 large onions, peeled and diced
2-3 sprigs fresh thyme
4 Russet potatoes, peeled and thinly sliced
2 cups heavy cream
4 Tablespoons blue cheese, crumbled
1 1/2 cups Gruyere cheese, grated
1/2 cup Parmesan cheese, grated
salt and freshly ground pepper

Preheat oven to 375F.
Heat oil in a large saute pan over medium heat. Add onions and thyme and cook slowly until caramelized (20-30 minutes). Remove thyme stems.
Combine potatoes and cream in a 2 quart saucepan and bring to a boil. Remove from heat and cool to warm, about 30 minutes.
Butter a 9x13 inch baking pan and place a layer of potatoes on the bottom and season with salt and pepper. Spoon 1/3 of onions over top and sprinkle with 1/3 of each cheese.
Repeat the layers to make 3, topping with remaining cheese.
Pour reserved cream evenly over top and cover with foil.
Bake 50-60 minutes or until potatoes are soft and cream is absorbed.
Remove foil and bake until top is golden brown.
Let stand 10-15 minutes before serving.