1 Tablespoon unsalted butter, room temperature
4 pounds of Yukon gold potatoes, peeled and sliced 1/4-inch thick
2 large cloves garlic, peeled and cut in half
6 cups milk
1 Tablespoon kosher salt
1 cup heavy cream
1/4 cup Dijon mustard
1/2 cup all-purpose flour
1 1/4 teaspoon kosher salt
1/4 teaspoon black pepper
2 bunches spinach, stems removed and washed thoroughly
8 ounces Gruyère cheese, grated on the medium-large holes of a box grater
1 Tablespoon yellow or brown mustard seeds
Preheat oven to 375ºF.
Brush a shallow, oval baking dish (about 12 by 7 inches) with butter.
Potatoes: In a stockpot, combine the peeled and sliced potatoes, garlic, milk and 1 Tablespoon of salt. Bring to a boil, then reduce heat, and let simmer until potatoes just begin to get tender but are not breaking apart, about 8 minutes. Drain potatoes, reserving 2 cups milk. In a large bowl, combine reserved milk with heavy cream then whisk in mustard and sprinkle in the flour slowly while whisking. Add 1 1/4 teaspoons kosher salt and 1/4 teaspoon pepper.
Spinach: Bring a large pot of heavily salted water to a boil. Place a large handful of spinach in the water and let wilt for 10 seconds. Remove with a skimmer and press between two plates to remove the excess liquid. Remove as much of the liquid as possible. Coarsely chop the spinach.
Assemble the gratin: Grate the cheese. Layer half of potatoes in the bottom of the baking dish. Scatter half of the cheese over the potatoes. Add all of the spinach on top of the cheese. Layer the remaining potatoes on top, covering the spinach completely. Slowly pour cream mixture over potatoes so that the liquid comes almost to the top of the potatoes. Scatter the remaining cheese over the top of the gratin. Sprinkle evenly with mustard seeds.
Cooking: Bake until potatoes are completely soft when pricked with a fork and cream is bubbling around the edges, about 20-25 minutes. Turn oven to broil. Adjust oven rack to top setting. Transfer gratin to upper rack and broil until top is golden brown, about 3 minutes. Let cool for five minutes before serving.
Recipe created by Parties That Cook®