Combine milk and flour in a heavy nonstick pan over medium high heat. Mix well. Add frozen vegetables and pasta. Bring to a boil, stirring occasionally. Cover pan, reduce heat to low and simmer 3-5 minutes, or until vegetables are crisp-tender and sauce is slightly thickened, stirring occasionally. Stir in tuna. Cover and cook over low heat 1 minute or until thoroughly heated. Sprinkle with croutons before serving.