Preheat the oven to 175°C. Spray a casserole dish with nonstick spray. Drain the macaroni, and then return it to the pot and keep warm. Combine the cauliflower, milk, cheeses, sour cream, mustard, salt, pepper, egg whites and peas in a large bowl and mix well. Stir the mixture into the warm macaroni, letting the residual heat combine the pasta and sauce. Pour into the casserole dish and evenly sprinkle the breadcrumbs on top.
Bake until the filling is hot and the top is golden, 35 to 40 minutes.