Place potatoes in a large stockpot; cover with stock. Bring mixture to a boil; cook 4 minutes. 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.
Heat a medium saucepan over medium heat. Add oil to pan. Sprinkle flour over oil; cook 1 minute, stirring constantly with a whisk. Add garlic; cook 2 minutes, stirring frequently. Combine milk and reserved 1 cup cooking liquid. Gradually pour milk mixture into flour mixture in pan, stirring constantly with a whisk. Add thyme sprigs to pan. Bring mixture to a boil; cook 4 minutes or until slightly thick, stirring frequently. Remove from heat. Strain mixture through a fine-mesh sieve over a bowl, reserving sauce; discard solids. Stir 1/2 teaspoon salt, pepper, and Gruyère cheese into sauce.
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. Sprinkle evenly with remaining 1/4 teaspoon salt, 1 tablespoon chives, and chopped thyme; pour remaining sauce over potato mixture. Sprinkle with Parmigiano-Reggiano cheese. Bake at 350° for 1 hour or until potatoes are tender when pierced with a knife.
Remove gratin from oven. Preheat broiler to high.
Place gratin in oven. Broil gratin 3 minutes or until browned. Sprinkle with remaining 1 tablespoon chives.
This dish was a big hit. I agree with others that it was a bit time consuming, but I did everything except the final baking step several hours ahead. It stayed very hot for a good 45 minutes after it came out of the oven. I would definitely make it again.
ok a couple of short cuts instead of cooking the potatoes first I put them in the casserole raw and then did the sauce with a cup of chicken broth and then poured it over the potatoes just like the directions said to then I simple cooked the potatoes longer and got the same result it just was a time saver on the prep end and timed out perfectly with my roast chicken
This was really good- though it was a lot of work. I would make it again for a holiday or company. Next time though I think I would probably make most of it ahead and refrigerate until it was time to bake it otherwise it would take too much time away from guests.
My family loved these potatoes on Christmas Eve. I did use the Gruyere. I had to make two casseroles, so after I strained the broth off the first batch of potatoes, I used it with additional broth to cook the second batch of potatoes so that I wouldn't be wasteful. In the future, I think I might save the broth and use it to make a soup or stew the next day. The first reviewer was right in saying it was rich and creamy. I was surprised that the potatoes cooked so quickly in the broth. Be sure to time them so they don't fall apart. The dish was a bit time-consuming, but it resulted in a dish that was beautiful and delicious!
The gratin had a nice delicate flavor. It was more work than I thought it would be. With that said, I decided to add this to my Thanksgiving menu instead of a sweet potato casserole, and I have to say, I missed the sweet potato casserole/:-(
This was very good. I used cheddar cheese as Gruyere was too expensive. This was a lot of work and mess (many pots to clean) so it may not become a regular dish, but it was delicious. I used a mandolin to slice the potatoes and I did like the two-tone effect with the sweet potato.
These were fantastic! Very rich & creamy. My husband & toddler both ate them up. Made some changes though. Used only unpeeled red potatoes & the medium cheddar I had on hand instead of Gruyere. Didn't strain either time (thought that was a bit ridiculous since there were hardly any solids).
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