Cook noodles according to package directions, adding the peas the last 2 minutes of cooking. Drain and return to pan.
Meanwhile, in a medium saucepan melt butter. Add onion, garlic, and black pepper; cook and stir over medium heat about 3 minutes or until onion is just tender. Stir in flour until blended. Add 1 1/4 cups milk all at once. Cook and stir until thickened and bubbly. Remove from heat; add tuna and 1/4 cup of the cheese.
Pour tuna mixture over noodles and peas in pan. Toss gently to combine. If desired, stir in additional milk, 1 tablespoon at at time until it reached desired consistency. Season to taste with salt. Divide among four shallow bowls. Drizzle with lemon juice. Sprinkle with remaining cheese and additional pepper.