Spinach Potato Gratin

I saw this recipe on the Chew and served it to friends. It was soooooooo good. The only change I made was to use a 10 oz package of chopped spinach because that's what I had. it came out great. My friends all wanted the recipe so I thought I'll share it here also.

• 2 pounds Yukon Gold Potatoes
• 8 ounces Frozen Spinach, Thawed
• 1 1/2 cups Whole Milk, warm
• 1 cup Heavy Cream, warm
• 1 Onion, Finely chopped
• 4 tablespoons butter
• 1/4 cup Flour
• 1/8 teaspoon Freshly Grated Nutmeg
• Dash Hot Sauce
• Salt and freshly grated black pepper
• 1 cup Gruyere, shredded
• Preheat oven to 350°F.
• Peel the potatoes and slice them thinly, about 1/8 inch thick. Place them in a bowl filled with cold water to prevent them from browning.
• Squeeze small handfuls of spinach to remove as much moisture as possible, then coarsely chop.
• In a large heavy-bottomed pot over medium heat, melt the butter. Add the flour and stir to combine. Cook the flour mixture for 2-3 minutes, stirring continuously. Slowly pour in the milk and cream and whisk to combine. Continue to cook, stirring continuously, until the sauce begins to thicken (about 5 minutes). Remove from heat and season with nutmeg, a few dashes of hot sauce, salt and pepper. Add in the chopped spinach and stir until combined.
• Drain the potatoes well and add them to the pot with the creamed spinach. Stir everything together being sure the potatoes are not stuck together and everything is evenly coated. Pour the mixture into a buttered gratin dish and top with gruyere cheese.
• Place the gratin dish on a baking sheet and bake for about 1 hour and 15 minutes, until the top is golden brown and the potatoes are cooked through.
• Remove from oven and allow to cool for 5-10 minutes. Serve warm.
  
  Tips
  - If you don't have frozen spinach you can replace it with sautéed spinach or kale.
  - Be sure to slice the potatoes thinly and evenly so that they evenly cook.
  - If you make the gratin the day before, fully cook it, let it cool to room temperature and refrigerate overnight. The next day, bring to room temperature and warm in the oven.