1-First Step: Mise en place and prep (10-20 minutes)
Preheat your oven to 350°F and lightly coat a 9×13-inch baking dish with cooking spray. Having the baking dish greased before you start keeps things moving once the layers are ready.

Peel and slice 3 pounds of sweet onions crosswise into 1/4-inch-thick rings. Separate the rings into individual pieces and place them in a large bowl. Tossing the rings apart now saves time during assembly and helps them cook evenly.

2-Second Step: Season the onions
In the large bowl with the onion rings, add 1 1/2 teaspoons kosher salt, 1 teaspoon dry mustard powder, 1 teaspoon garlic powder, 1 teaspoon smoked paprika, and 1/4 teaspoon cayenne pepper if you want a little heat. Toss until every ring looks evenly coated.

The salt draws a bit of moisture out of the onions so they soften while baking, while the mustard and smoked paprika add savory depth that complements the cheeses.

3-Third Step: Assemble the layers
Layer half of the seasoned onion rings in the prepared baking dish so they cover the bottom evenly. Sprinkle half of the shredded mild cheddar and half of the shredded smoked Gouda (2 ounces cheddar and 2 ounces Gouda if dividing equally) over the onions.

Add the remaining onions in an even second layer, then top with the rest of the shredded mild cheddar and smoked Gouda. Even layering ensures every portion gets melted cheese and sweet onion in each bite.

4-Fourth Step: Finish with Parmesan and butter
Sprinkle 1 1/4 ounces of grated Parmesan cheese evenly over the top of the casserole. Scatter the 4 tablespoons of diced unsalted butter across the surface in small bits so they melt into pockets and help the top brown.

5-Fifth Step: Cover and bake
Cover the dish tightly with aluminum foil and bake at 350°F for about 45 minutes. The foil traps steam so the onions soften thoroughly and the cheeses start to melt and mingle.

6-Sixth Step: Uncover and finish baking
After 45 minutes, remove the foil and continue baking for about 30 minutes. You want the casserole to be bubbly around the edges and for the cheese to have some golden brown spots on top. This step develops color and a little crispiness while keeping the inside creamy.

7-Final Step: Rest, garnish, and serve
Let the casserole sit for 8 to 10 minutes after removing it from the oven so it firms up slightly for easier serving. Garnish with 2 tablespoons of finely chopped fresh parsley if you want a pop of green and freshness.

Serve Tennessee Onions warm as a side with smoked brisket, roasted chicken, or as part of a holiday spread. If you plan to serve alongside a heavier pasta or baked dish, this casserole’s sweet and smoky profile complements well; try pairing it with a baked pasta like sturdy baked ziti side for a hearty meal.