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Preheat oven to 350°F (175°C) and grease a 9×13-inch baking dish.
Mix thyme, parsley, garlic powder, oregano, salt, pepper, and mustard powder in a bowl.
Peel and slice onions into 1/4-inch rounds, separate into rings, and coat with the spice mix.
Arrange onions in the baking dish, dot with butter, and top with cheeses.
Bake for 35-40 minutes until onions are tender and cheese is melted and bubbly. Broil for 2-3 minutes for a crispy top.
Let cool for 5 minutes before serving.
Prep Time: 5 mins | Total Time: 40 mins | Servings: 8
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