How to Make It
Preheat oven to 350°F. Preheat Grill.
In a medium mixing bowl combine the half and half, fresh horseradish, fresh thyme leaves, half the salt and pepper, mix well, set aside.
Wash the sweet potatoes under cold running water, peel with a vegetable peeler. Take the sweet potatoes and vidalia onion, place on a medium cutting board, being careful take a chef knife and cut 1/4 inch thick round slices, place slices on a plate. Brush the sweet potatoes and onions slices with the canola oil, season with half of the salt and pepper, individually place the slices onto the grill, grill on each side until slightly charred and half cooked, place the grilled sweet potatoes and vidalia onion slices on paper towel lined plate to absorb in excess juices.
Place 4 to 6, 5 inch long gratin dishes onto a baking tray, brush the dishes with melted butter. Sprinkle 2/3 cup of the grated gruyere on the bottoms of the gratin dishes evenly. Layer 1/3 of the grilled sweet potatoes and vidalia onions on top of the gruyere evenly. Repeat this process 2 more times to create 3 even layers. Evenly pour the half and half mixture over the gratin. Finish top with extra grated gruyere cheese. Cover the gratins with foil and place into the 350 oven, bake for 35 to 40 minutes covered.
Uncover the gratins and bake 15 to 20 more minutes until the Gratin is golden brown and the sweet potatoes are tender.
Remove the gratins from the oven and allow to rest for 15 minutes prior to serving.