Momosita's Sweet Potato Gratin

I modified two recipes - one for a sweet potato gratin and one for a regular potato gratin- to come up with this lovely dish. The dijon mustard gives it a wonderful flavor, however; if you don't like mustard, you can omit it and follow the rest of the recipe as written.

• Ready In: 1 hr 40 mins
• Serves: 8-10

Ingredients

• 2  large sweet potatoes, sliced 1/8-inch
• 3 -4  small russet potatoes, sliced 1/8-inch
• 1  medium onion, diced
• 4  tablespoons butter
• 1  cup heavy cream
• 1  cup milk
• 4 -5  tablespoons Dijon mustard
• 1 1⁄2 teaspoons salt
• 1⁄2 teaspoon pepper
• 1⁄4 teaspoon cayenne pepper
• 1 1⁄2 teaspoons thyme
• 1  cup panko breadcrumbs
• 1⁄4-1⁄2 teaspoon sage
• 3⁄4 cup sharp cheddar cheese, grated

Directions

1. Heat the oven to 400°F.
2. Melt butter in a medium frying pan over medium-low heat. When it foams, add onion. Slowly cook until soft and translucent; set aside.
3. Cut into potatoes into 1/8" slices with a mandolin, sharp knife, or food processor with slicer attachment.
4. Whisk the cream, milk, mustard, cayenne and thyme in a medium bowl and set aside.
5. In a 9x13" baking dish, layer 1/2 potato slices on the bottom, 1/2 cream mixture, add salt and pepper, 1/2 of the cheese. and ½ of the onions. Repeat, layering the rest of the potato slices, cream mixture, salt and pepper, and the rest of the onions.
6. Cover with foil and bake 50 minutes to one hour (the potatoes should be cooked through).
7. Sprinkle panko or stuffing mix evenly over gratin.
8. Lightly sprinkle sage over the gratin.
9. Sprinkle the rest of the cheese over the gratin.
10. Bake uncovered, about 10-20 minutes, until golden brown.
11. Let it sit for 10 minutes or until the cream sauce thickens.