In 10-inch skillet, cook beef and onion over medium-high heat 5 to 7 minutes, stirring frequently, until beef is thoroughly cooked; drain.
Stir in milk, hot water, and uncooked pasta and sauce mix (from Hamburger Helper box). Heat to boiling, stirring occasionally. Reduce heat; cover and simmer 10 minutes, stirring occasionally.
Arrange tomato slices on top; sprinkle with croutons. Place 5 of the cheese halves around edge of skillet; place remaining cheese halves in center. Cover and cook 2 minutes. Remove from heat; let stand covered 5 minutes (cheese will continue to melt and sauce will thicken as it stands).