In a large pot, heat 2 1/2 cups (625 mL) of the milk and water over medium heat until steaming. Stir in pasta. Cover, reduce heat to medium-low and simmer for 10 minutes, stirring once. Add broccoli on top of pasta, cover and simmer for 5 minutes or until pasta is tender and broccoli is tender-crisp.
Stir ricotta, lemon zest, lemon juice and pepper into pot until blended. Stir in enough of the remaining milk to moisten pasta to desired consistency. Gently stir in tuna, just until heated through.