Directions

• Preheat oven to 350.
• In a large skillet, heat 2 tablespoons of butter over medium-high heat. Add onions and cook until lightly browned, about 5 minutes.
• Lower heat to medium and continue cooking until onions caramelize, about 20 minutes.
• Meanwhile, in another large skillet, heat 3 tablespoons of butter over medium heat. Add bread, salt, and pepper and cook until light golden brown, about 8 minutes.
• In a large bowl, whisk together eggs, half and half, mustard, cayenne, salt, and pepper.
• Stir in the onions, bread, ham, and 1 cup of Gruyere.
• Butter a 9x13 inch casserole dish, and transfer the mixture to the dish.
• Top with a single layer of tomatoes, the rest of the Gruyere, and the Parmesan.
• Bake until the top is golden brown and the casserole is set (it doesn’t jiggle when shaken), about 40 minutes.
• Top with parsley and serve.