Heat oil on MEDIUM in large braising pan. Brown monkfish lightly on all sides, 4-6 minute Remove monkfish; set aside. Discard all but 1 Tbsp oil.
Lower heat to MEDIUM-LOW; add garlic and onions. Cook 2-3 min, stirring frequently. Add tomatoes and olives; cook 2-3 minute Add broth and marjoram sprig and return monkfish to pan. Bring to simmer and cover; cook 20-25 min, until fish reaches internal temp of 140 degrees. Check by inserting thermometer into thickest part of fish.
Transfer fish to clean plate; continue to simmer sauce 5 min, until desired thickness. Season to taste with salt and pepper. Pour sauce over fish.