Potato fennel gratin

Ingredients

2 fennel bulbs
1 yellow onion, thinly sliced
2 tbls olive oil
1 tbls unsalted butter
2 lbs russet potatoes (4 large)
2 cups heavy cream
2 1/2 cups grated gruyere (1/2 lb)
1 teas kosher salt
1/2 teas freshly ground pepper

Instructions

Preheat oven to 350 deg F
Butter the inside of a 10 x 15 x 2 baking dish.
Remove the stalks and cores and thinly slice fennel. Saute the fennel and onion with butter on medium heat for 15 mins, until tender.
Peel the potatoes then thinly slice them by hand or with a mandoline. Mix the sliced potatoes in a large bowl with 2 cups of cream, 2 cups of cheese, salt and pepper. Add the sauted onion and fennel and mix well. Pour into the baking dish, press down. Combine the remaining 2 tbls of cream ans 1/2 cup gruyere and sprinkle on top. Bake for 1 1/2 hrs until the potatoes are very tender and the top is browned. Allow to set for 10 mins and serve.