In 3-quart saucepan, melt butter over medium heat. Add onion; cook and stir until tender. Reduce heat to low. Stir in flour, mustard and pepper; cook and stir until mixture is smooth and bubbly.
Gradually stir in milk and broth. Stir in frozen vegetables and marjoram. Heat to boiling over medium heat, stirring occasionally. Reduce heat to low; cover and simmer 3 to 5 minutes or until vegetables are crisp-tender.
Add cheese and tuna; heat, stirring gently, until cheese is melted.