If you’re also making grilled cheese, start soup on medium low before prepping sandwiches (Turn down to low if it starts to boil at all while sandwich is cooking). Start on medium and cook for 10 minutes if serving alone or as starter. Turn down to low if starts to boil.
Put soup in pot. Add milk and water (1/3 water and 2/3 milk if using 2% or skim milk). Use whisk or fork to get lumps out. Add diced tomatoes, if using, and stir. Add garlic, onion powder, Italian season (or parsley) and pepper. Whisk in until thoroughly mixed in. Stir occasionally.
Serve in bowl, soup mug or coffee mug (not teacup) and top with desired amount of parmesan. If you want to get fancy, you can top with a little bit of fresh Italian parsley