In a pot of salted water, cook cauliflower and macaroni al dente according to the package directions. Drain and set aside.
In a large skillet, heat two tablespoons olive oil over medium-high heat. Add onions and cook until tender, stirring frequently. Add cooked macaroni and cauliflower. Stir in sour cream, milk, cheese, and mustard. Season the macaroni with salt and pepper. Stir well until cheese is melted. Transfer to a casserole dish.
In a small bowl, mix bread crumbs, parsley, and one tablespoon olive oil. Spread over the macaroni and cheese to cover.
Bake at 350 degrees Fahrenheit for 30 minutes or until the top is golden brown.