In a large pot boil water and add the cauliflower florets. Boil for 2 minutes until tender. Drain, pour in a casserole dish, then add the frozen green peas and corn. Set aside.
Preheat an oven to 400 F degrees.
Heat the butter over medium heat in a small skillet. Add the flour, whisk to combine. Slowly whisk in milk until the flour is dissolved. Add grated Parmesan cheese and stir until melted. Season with salt and pepper. Turn off the heat and set aside the mixture.
In a medium bowl, beat the egg, and pour it into the slightly cool mixture, mix well.
Toss in the cauliflower and stir together in a casserole dish.
Sprinkle with the mozzarella on top.
Bake for 25 minutes or until the top is golden.
Garnish with parsley, and enjoy!