In 12-inch skillet, heat milk, hot water, butter, uncooked pasta and sauce mix (from Tuna Helper box), carrots and tuna to boiling over high heat, stirring occasionally.
Reduce heat; cover and simmer 5 minutes, stirring occasionally.
Stir in peas and mushrooms. Cover; simmer about 5 minutes or until vegetables are tender. Remove from heat; let stand covered 5 minutes (sauce will thicken as it stands).