Make the dressing: In a bowl or jar, whisk together mayo, vinegar, water, olive oil, grated cheese, garlic powder, Italian seasoning, sugar, salt, and pepper. Chill for 10–15 minutes if you have time.
Prepare the vegetables: Chop the romaine lettuce, slice the onion thinly, and halve the cherry tomatoes.
Assemble the salad: In a large bowl, combine lettuce, onions, tomatoes, black olives, and pepperoncini.
Add the croutons and cheese: Sprinkle croutons and grated Parmesan over the top.
Drizzle with dressing: Pour desired amount of dressing over the salad and toss gently to combine. Serve immediately!