In a large saucepan, mix broth with 1 can of water. Add potatoes and simmer 10 to 15 minutes until tender. Remove cooked potatoes from broth, reserving liquid. Puree cooked potatoes with 1/4 cup broth. Add tuna, vegetables, seasonings and pureed potatoes to remaining broth in saucepan. Simmer 8 to 10 minutes until fish flakes easily. Stir in milk and heat to serving temperature without boiling. Sprinkle with parsley just before serving.