• Preheat oven to 400°Fahrenheit (204 degrees Celsius).
• Lightly grease a 9x13” baking dish with butter, set aside.

Make the Cheese Sauce

• Melt the butter in a medium saucepan over medium heat.
• Add flour, salt, garlic powder and pepper and whisk to combine. Cook for about 1 minute, or until butter bubbles and lightly browns.
• Add milk and heavy cream and whisk to combine.
• Cook the sauce over medium heat, whisking often, until it has thickened (about 2-3 minutes).
• Once thickened, turn off the heat, add the cheese and whisk until melted. Set aside. (it is thick).

Make the Sweet Potato Gratin

• Place ⅓ of the sweet potatoes on the bottom of the prepared pan.
• Top with half of the onion slices, then lightly sprinkle with salt and pepper.
• Top onions with the second third of the sweet potatoes, then top that layer of sweet potatoes with remaining onion slices and salt and pepper
• Then, put the final third of the sweet potatoes on top, and season with salt and pepper.
• Next, pour the cheese sauce over the top of the potatoes.

Bake, Cool & Serve

• Cover the baking dish with aluminum foil and bake in the preheated oven for 50 minutes to 1 hour, or until the sweet potatoes are soft when pricked with a fork (but not mushy).
• Once the sweet potatoes are soft, remove the foil and sprinkle the top with 1 cup of cheddar cheese and 1 cup gouda cheese.
• Bake on the top rack of your oven, uncovered, for an additional 8 to 10 minutes until the cheese is melted, bubbly and starting to turn lightly golden brown, the filling should be bubbling, and the potatoes should be tender.
• Remove from the oven, let cool slightly (uncovered) and serve warm.