3 1/2-4lbsrusset potatoes (peel intact)1/8 inch slices
4Tbspsalted butter
1smallonionfinely diced
3Tbspall purpose flour
1tspsalt
1tspblack pepper
1/2tspground mustard
1/8tspground nutmeg
1largeclove garlicfinely minced
1Tbspfresh thyme leavesplus additional for garnishing
2cupswhole milk or half and half
2cupsshredded Gruyere cheesedivided use
1/4cupgrated Parmesan cheese, plus 2-3 Tbsp for the top
Instructions
Preheat oven to 375°F. Butter an 11 x 8 inch (2.5 quart baking dish) or similar size baking dish with 1 Tablespoon of butter. Set aside.
Thoroughly clean potatoes and using a mandolin slice 1/8 inch slices.
To make sauce: In a heavy bottomed saucepan, melt butter over medium heat. Add onion, cooking for 1-2 minutes or just until softened. Add garlic, cook for 1 minute. Add flour, stir until butter is absorbed.
To the flour add salt, black pepper, ground mustard and nutmeg. Gradually whisk in milk. Add thyme. Bring to a light bubble then immediately lower the heat to simmer for 3-5 minutes, stirring occasionally.
Remove from heat. Add 1 cup Gruyere and 1/4 Parmesan cheese to sauce, Mix until melted.
To assemble: Layer 1/2 potato slices fanned and overlapping in baking dish. Spread 1/2 sauce evenly over potatoes.
Repeat remaining potatoes, spread with sauce. Sprinkle top with remaining Gruyere and sprinkle with 3 Tablespoons of Parmesan.
Cover with foil and bake for 50-60 minutes. Uncover, increase oven temperature to 400 °F. Bake for another 20 minutes or until the top is golden and the potatoes are fork tender.
Let stand 5-10 minutes of the sauce to thicken then serve garnished with fresh thyme.