Preheat the oven to 375 degrees F. Coat a 9-inch pie plate with nonstick cooking spray.
In a large mixing bowl, whisk together the eggs, milk, cornstarch and ground mustard.
Stir in the drained tomatoes, mushrooms, ham and scallions. Season liberally with salt and pepper. Stir in the croutons before transferring the mixture to the prepared pie plate. Top with the Cheddar and Parmesan.
Bake until the center is set, about 30 to 35 minutes. Let cool on a wire rack. Serve warm or at room temperature.
You can use the remaining half can of diced tomatoes in Chicken Pozole. Smaller sized croutons work best.