Ingredients

• 6 tablespoons unsalted butter, plus more for the mini skillets
• 1 large onion, thinly sliced
• kosher salt and freshly ground black pepper
• 4 cups 1/2-inch bread cubes (about 8 ounces, crusts removed)
• 8 ounces cremini mushrooms, sliced
• 1 1/4 cups half-and-half
• 1 teaspoon dijon mustard
• pinch cayenne pepper
• 8 large eggs
• 8 ounces ham, diced
• 2 cups shredded gruyere (about 6 ounces)
• 1 pint grape tomatoes, halved
• 1/4 cup grated parmesan