In a large kettle, melt the margarine over medium heat. Add the onion and saute until tender, about 4 minutes. Add the water and bouillon cubes; stir to dissolve. Bring to a boil. Add the frozen broccoli and simmer for 5 minutes. Add the noodles and simmer for 5 minutes. Add the milk and Velveeta and simmer until the cheese is melted. Season to taste with pepper.