Preheat your oven to 400°F (200°C).

Melt 3 tablespoons of butter in a medium saucepan over medium heat, then add 2 minced cloves of garlic and ½ finely diced red onion. Cook for about 5 minutes until softened.

Sprinkle in 2 tablespoons of all-purpose flour and cook for an additional minute while stirring constantly. Gradually whisk in 1 cup of heavy cream and ½ cup of half and half, cooking for 3-4 minutes until sauce thickens.

Remove from heat and stir in ½ cup of shredded Asiago cheese, 2 tablespoons of fresh rosemary, 1 teaspoon of smoked paprika, ½ teaspoon of ground sage, and season with 1 teaspoon of kosher salt, ½ teaspoon of black pepper, and a hint of nutmeg. Allow the sauce to cool to room temperature.

Wash, peel, and slice 3-4 medium sweet potatoes into thin rounds (approximately ⅜ inch thick). Arrange the slices in a greased 9x13-inch baking dish.

Stir half of the shredded Colby Jack and smoked Gouda cheese into the sauce. Pour the sauce evenly over the sweet potatoes, ensuring each slice is coated.

Cover the baking dish with aluminum foil and bake for 45-50 minutes until sweet potatoes are tender.

After the initial bake, remove the foil, sprinkle remaining cheeses on top, and return to the oven for an additional 15-20 minutes until the top is golden brown.

Let the Sweet Potato Gratin rest for about 10 minutes before serving. Garnish as desired.