Preheat the oven to 350 degrees F (175 degrees C). Grease an 8 inch square baking dish.
Melt the butter in a large skillet over medium heat. Add onions; cook and stir until tender, about 5 minutes. Transfer to the prepared baking dish. In a medium bowl, stir together the beef broth and flour until smooth. Mix in the cream and season with salt and pepper; pour over the onions. Sprinkle Swiss cheese and Parmesan cheese over the top.
Bake for 25 to 30 minutes in the preheated oven, until cheese is golden brown and the sauce is bubbly.
recipepes.comOnions Au Gratin, recipePT15MPT1H5455 calories