Preheat the oven to 350 degrees F. Spray a 9x13 inch glass baking dish with nonstick spray.
Slice the English muffins into small bite sized pieces. Scatter them into the bottom of the prepared pan.
In a large skillet, heat the olive oil over medium high heat; add the onion and bell peppers and sauté until softened, about 5 to 6 minutes. Set aside to cool.
Sprinkle the ham over the muffin cubes. Scatter the onion/bell peppers over the ham. Sprinkle the shredded cheese evenly over the top.
In a medium bowl, whisk together the eggs, milk, mustard, salt and pepper. Distribute the egg mixture evenly into the dish.
Bake until set and golden, about 40 minutes. Let stand for 10 minutes before serving.
Notes
If you'd prefer a little spice, sub a can of mild chopped chiles in place of the green bell pepper.