In 12-inch skillet, stir together uncooked pasta and sauce mix from Tuna Helper box, water, milk, butter and garlic powder. Heat to boiling over medium heat, stirring occasionally. Stir in tuna, spinach and tomatoes.
Reduce heat to medium-low; cover and cook 13 to 15 minutes, stirring occasionally, until pasta is tender. Stir in lemon juice; sprinkle with cheese.