Wash the cubed beef well, pat them dry with a toe, and set aside in the fridge. (Please don't skip this step. It's an important part of the process for tenderizing the beef).
Finely chop the onions in a blender or food processor, until almost puréed.
Transfer the onions to a heavy pot and cook on medium heat until they are day and have turned a reddish-brown color. Add the water and Berberi. Cook an additional 30 minutes, stirring periodically, adding more water if needed so the sauce doesn't get dry (this process allows for the berberi to become less bitter).
Add the meat to the sauce and turn the heat down to low. Simmer for 45 minutes to an hour, adding a little water if needed, to prevent the onions and meat from sticking the bottom of the pan.
After an hour or so of simmering, add the butter, salt, and more water if the sauce is getting dry. Simmer another hour.
Just before serving, stir the freshly minced garlic into the sauce. Serve with Injera.