1. Rinse beef, pat dry, and cut into 1-inch chunks. In a 5- to 6-quart pan, combine beef, onions, and 1 cup water. Cover and bring to a boil over high heat; reduce heat and simmer 30 minutes. Uncover, turn heat to high, and stir often until liquid evaporates and meat and onions are lightly browned, 5 to 7 minutes. Spoon out and discard any fat.
2. Add to pan the curry powder, mustard seed, garlic, and turmeric; stir until spices are more fragrant, about 1 minute. Add broth, tomatoes, chilies, and ginger; stir to free browned bits. Return to a boil, cover, reduce heat, and simmer until meat is very tender when pierced, 2 to 2 1/2 hours.
3. Put banana, chutney, coconut, and cucumber yogurt sauce each in separate small bowls.
4. Ladle beef curry over rice on plates. Add banana, chutney, coconut, cucumber yogurt sauce, and salt to taste.