Preheat your oven to 400°F (200°C) and prepare a 9x13-inch baking dish by greasing it lightly with butter or cooking spray.

In a large saucepan, melt the butter over medium heat. Add minced garlic and chopped red onion, sautéing until the onion is soft and translucent—about 5 minutes.

Sprinkle in the all-purpose flour, stirring continuously for about 1 minute. Gradually whisk in heavy cream and half and half, cooking until it thickens slightly—around 3 to 4 minutes.

Once thickened, remove the saucepan from the heat and let it cool a bit before stirring in grated Asiago cheese, chopped rosemary, smoked paprika, ground sage, kosher salt, and black pepper, along with a pinch of freshly grated nutmeg.

Slice the sweet potatoes into uniform ⅜-inch thick rounds using a mandoline slicer or a sharp knife.

Layer the sweet potato slices in the baking dish, covering the bottom with an even layer. Pour half of the creamy sauce over the potatoes.

Continue layering the remaining sweet potato slices on top and pour the rest of the creamy sauce over the final layer.

Sprinkle with shredded smoked Gouda cheese and shredded Colby Jack cheese.

Cover tightly with aluminum foil to trap steam, and bake for 45 to 50 minutes until sweet potatoes are tender.

Remove foil and bake uncovered for an additional 15 to 20 minutes until the top is golden brown and bubbly.

Once baked, let the gratin rest for about 10 to 15 minutes before serving.