Place trimmed lamb cubes in a large bowl. Season meat with ground coriander, salt pepper, and paprika. Toss to coat.
In a large, non-stick skillet, sear the lamb cubes, in small batches. Place seared meat in a bowl and set aside. Leave any drippings in the skillet.
Add the onions to the skillet and cook in the drippings until they are soft and slightly caramelized.
Place seared meat and onions in a large pot.
Dilute the tomato paste in 2 cups of water and pour over the meat. If the sauce looks too thick, stir in more water, a little at a time. Continue to add more water, as needed, during cooking.
Stir in the bouillon cube and sugar.
Bring to a boil, then reduce heat to low. Cook, with the pot lid slightly tilted, for 1 ½ to 2 hours, or until the meat is fork-tender. Stir occasionally, and continue to check the level of liquid, adding more as needed.
Just before serving, adjust seasings, if necessary and stir in the butter to mellow the acidity of the tomato sauce.
Serve with rice pilaf, bulgur pilaf, or noodles.