Potato Gratin with Fennel and Leek

Cheesy potatoes are already good.  Add fennel and leek and it's over the top!  This is undeniably a stand out side dish.

4 TBSP  butter
2 large leeks, rinsed well and thinly sliced
2 cups heavy cream
1 cup whole milk
1 TBSP kosher salt plus more to taste
1/2 tsp or more of fresh ground pepper
2 1/2 lbs Yukon gold potatoes, thinly sliced, do not rinse
2/3 cup grated Gruyere cheese
1/2 cup grated Parmesan cheese
3 medium fennel bulbs, fronds trimmed and bulbs thinly sliced
3 TBSP fresh thyme leaves, for garnish

In a very large skillet or dutch oven melt 2 TBSP butter, add leeks and fennel and saute until soft but not brown, about 4-5 minutes.  Add the cream and the milk to the pan until mixture starts to simmer.  Add salt and pepper.  Add sliced potatoes to the pot and cook for about 10-12 min.  Press the potatoes down so that they are covered by the liquid.  Don't stir the potatoes or they will break and turn to mashed potatoes. 

Preheat the oven to 375 degrees with rack in center of oven.  Generously butter a 13x9x2 inch baking dish.  Very gently stir in the cheeses reserving about 1/2 cup to sprinkle on top.    Transfer the potatoes to the baking dish and dot with remaining 2 TBSP of butter.  Cover with remaining cheese.  Bake for about 50-60 minutes or until the top is golden brown.  If the top starts to get too brown cover with a sheet of aluminum foil sprayed with cooking spray.  Top with  fresh thyme and a sprinkle of finishing salt.