Two-Potato Gratin

Ingredients

• 2 medium baking potatoes peeled cut into 1/4-inch-thick slices ( 3 cups)
• 0.3 teaspoon pepper black freshly ground
• 2 tablespoons canola oil 
• 2 quarts no-salt-added chicken stock (such as Swanson)
• 3 tablespoons flour all-purpose
• 2 tablespoons chives fresh divided chopped
• 1.5 teaspoons thyme leaves fresh chopped
• 2  garlic cloves crushed
• 0.8 cup aged gruyère cheese shredded
• 0.8 teaspoon kosher salt divided
• 1.5 cups milk 2% reduced-fat
• 0.3 cup parmigiano-reggiano cheese fresh grated
• 2 medium sweet potatoes peeled cut into 1/4-inch-thick slices ( 4 cups)
• 2  thyme sprigs 

Equipment

• bowl
• frying pan
• sauce pan
• oven
• knife
• whisk
• pot
• sieve
• baking pan
• broiler
• slotted spoon

Directions

1. Preheat oven to 35
2. Place potatoes in a large stockpot; cover with stock. Bring mixture to a boil; cook 4 minutes.
3. Remove from heat. Carefully remove potatoes from pot using a slotted spoon, reserving cooking liquid. Arrange potato slices in a single layer on a jelly-roll pan; set aside. Strain cooking liquid through a fine-mesh sieve over a bowl; reserve 1 cup cooking liquid. Discard solids and remaining cooking liquid.
4. Heat a medium saucepan over medium heat.
5. Add oil to pan.
6. Sprinkle flour over oil; cook 1 minute, stirring constantly with a whisk.
7. Add garlic; cook 2 minutes, stirring frequently.
8. Combine milk and reserved 1 cup cooking liquid. Gradually pour milk mixture into flour mixture in pan, stirring constantly with a whisk.
9. Add thyme sprigs to pan. Bring mixture to a boil; cook 4 minutes or until slightly thick, stirring frequently.
10. Remove from heat. Strain mixture through a fine-mesh sieve over a bowl, reserving sauce; discard solids. Stir 1/2 teaspoon salt, pepper, and Gruyre cheese into sauce.
11. Spread 1/2 cup sauce in bottom of a broiler-safe 2-quart ceramic baking dish coated with cooking spray. Arrange a single, flat layer of sweet potato and then baking potato slices over sauce. Over flat layer, alternate baking potato and sweet potato slices, in shingle-like fashion.
12. Sprinkle evenly with remaining 1/4 teaspoon salt, 1 tablespoon chives, and chopped thyme; pour remaining sauce over potato mixture.
13. Sprinkle with Parmigiano-Reggiano cheese.
14. Bake at 350 for 1 hour or until potatoes are tender when pierced with a knife.
15. Remove gratin from oven. Preheat broiler to high.
16. Place gratin in oven. Broil gratin 3 minutes or until browned.
17. Sprinkle with remaining 1 tablespoon chives.