Walnut-Sage Potatoes Au Gratin

I "auditioned" a dish for Passover this week... The recipe is from user Annacia on RecipeZaar, with minor gluten-free adjustments. I offered to review the recipe for her, thinking it might work if I had a vegetarian seder. It looks like I won't be having a vegetarian seder, but this dish is a sure winner. It might just be my new favorite potato dish, second only to kugel. It is so decadent and flavorful, and reheats very well in the oven or microwave.

I have to admit I was tempted to add a lot more cheese than the recipe calls for- And ultimately, I did add a bit more to the top of the casserole at the end, and then stuck it under the broiler for 30 seconds. I like to think I made up for this by using 2% milk instead of whole, but I suppose it doesn't work that way. Oh, well. It sure was good. Don't use American-style gruyere if you can help it - make sure to find a gruyere that is Swiss, otherwise use fontina or emmenthal or another good melting cheese with a little bit of bite to it.

To make a gluten-free roux for this dish I used arrowroot flour, a fine powder which works wonderfully as a flour substitute in gravies and cream sauces. You can try using other thickeners, but stay away from gluten-free flour mixes that contain nuts or other darker or coarser particles (for example, Pamela's Baking and Pancake Mix) as you will not achieve the smooth, white consistency you are going for with a white sauce. Cornstarch is not recommended since it becomes gummy when the dish cools or is refrigerated. I've had good luck with both Asian sweet rice flour (also called sticky rice flour or glutinous rice flour) and arrowroot, which are both finely ground and white in color. For Passover, I would try using heavy cream instead of 2% milk, and using potato starch as a thickener (adjust the quantities as necessary).

This is a terrific way to use one of my favorite new ingredients, walnut oil. It is well worth seeking out walnut oil for this recipe, because it has such a beautiful flavor. Trader Joe's is now carrying it, which means it's sure to become a staple in many kitchens within the blink of an eye. It looks like Gefen and Nutola both make kosher l'pesach walnut oil. This dish also shows off the complexity of fresh sage in a way that is lovely. The sage, walnuts, and walnut oil work together to make this a more sophisticated and less pedestrian gratin. Thanks, Annacia!

WALNUT-SAGE POTATOES AU GRATIN

6 medium russet potatoes (2 lb.)
1/2 cup chopped onions
5 garlic cloves, minced
3 Tbsp walnut oil
4 Tbsp potato starch (for Passover) or arrowroot flour
1/2 teaspoon salt
1/4 teaspoon pepper
2 1/2 cups 2% or whole milk
3 tablespoons snipped fresh sage
4 ounces Swiss gruyere cheese, shredded (1 cup)
1/3 cup broken walnut pieces
Fresh sage leaves (optional)

Peel potatoes and thinly slice. Use a knife or the thin slicing blade on the food processor. Slices should be no thicker than 1/4." Place slices in a bowl, submered in cold water to keep from browning.

In a medium saucepan on medium to medium-high heat, cook the onion and garlic in walnut oil until tender but not browned. Stir in flour, salt, and pepper, then quickly add all of the milk. Cook over medium heat, stirring constantly with a whisk until thickened and bubbly. Remove from heat; stir in snipped sage.

Grease a 2-quart round casserole (with glass cover) or a deep 2-quart baking dish. Layer half of the potatoes in casserole. Cover with half the sauce. Sprinkle with half the cheese. Repeat layering with the potatoes and sauce. Cover with lid or tented tinfoil.

Bake uncovered at 350 F for 40 minutes, then uncover and bake 25-40 minutes more or until potatoes are tender. Sprinkle remaining cheese and all of the chopped walnuts over top. Bake, uncovered, 5 minutes or until cheese is melted and bubbly. Let stand 10 minutes before serving. Garnish with remaining sage leaves.