Once water comes to rolling boil, salt water and stir in pasta. Cook 3 minutes and then pour in cauliflower pieces. Cook until pasta is al dente and cauliflower is soft. Drain and set aside.
In same pot over medium heat, saute onion in olive oil until soft, 5 minutes. Sprinkle in salt and pepper. Pour in alfredo sauce. Pour milk into jar of alfredo sauce, replace lid and shake to remove any excess. Uncap and pour into pot.
Sprinkle 2 cups of cheese into sauce and stir until cheese has melted and sauce is smooth.
Pour in cooked pasta, cauliflower and frozen peas. Stir until everything is well mixed and coated in sauce.
Splash in a little milk if sauce seems to thick. Smooth the top and sprinkle with remaining 1/2 cup cheese.
Remove from heat and cover to melt cheese OR stick pot under hot broiler for 1-2 minutes or until cheese is brown and bubbly. Serve hot.