In Dutch oven, cook spaghetti as directed on package omiting salt, adding peas during last 2 minutes of cooking. Drain in colander; cover to keep warm.
In same Dutch oven, mix milk, cheese product and pepper. Cook over medium heat, stirring frequently, until cheese is melted.
Gently stir in cooked spaghetti and tuna. Cook over medium heat, stirring occasionally, until thoroughly heated. If desired, sprinkle with grated Parmesan cheese.