Ingredients

Cooking spray
2 1/2 lb. sweet Vidalia onions (3 large or 4 medium), peeled
1 tsp. dried parsley
1 tsp. dry mustard
1 tsp. garlic powder
3/4 tsp. seasoned salt
1/2 tsp. dried oregano
1/2 tsp. kosher salt
1/4 tsp. cayenne pepper
1/4 tsp. freshly ground black pepper
1 cup shredded mozzarella
1 cup shredded sharp cheddar
1/2 cup finely grated Parmesan
4 Tbsp. unsalted butter, cut into cubes
1/2 cup low-sodium vegetable broth
Finely chopped fresh parsley, for serving

Directions

Set a rack in center of oven; preheat to 350°. Lightly grease a 13” x 9” baking dish with cooking spray.

Lay an onion on its side on a cutting board. Using vertical slices, cut onion into 1/4″-thick rounds. Separate rounds into individual rings. Re-stack rings, starting from the largest piece to the smallest (mimicking the original shape of the rounds) in prepared dish. Repeat with remaining onions for a total of 4 stacks.

In a small bowl, mix parsley, mustard, garlic powder, seasoned salt, oregano, kosher salt, cayenne, and black pepper. In a medium bowl, toss mozzarella, cheddar, and Parmesan.

Sprinkle onions with seasonings and gently toss to coat. Dot onions with butter. Add broth to bottom of pan and tightly cover with foil.

Bake onions until tender, 75 to 90 minutes (the centers should be easily pierced with a fork or a metal cake tester). Remove from oven and sprinkle with cheese mixture.

Continue to bake until cheese is melted and bubbling around the edges, 5 to 7 minutes.

Turn on broiler. Broil, watching closely, until cheese is browned in a few spots, 1 to 2 minutes more. Top with parsley.