Heat garlic in oil until lightly golden. Add tomatoes and their juices and heat for 20 minutes over low heat until tomatoes are really tender and almost melted. Crumble tuna into smaller pieces and add to tomato sauce. Simmer just to warm the tuna through; you don't want to boil the tuna. Transfer spaghetti to serving bowl and mix in tuna, tomatoes, parsley; and salt & pepper.