Put sliced onion on the bottom of the pot with beef bone marrow pieces on top of those. Drop minced garlic around pieces with a sprinkle of salt and pepper on top of everything. (if you want to make stock as well I would add large chunks of celery and carrots + a sprig of thyme and parsley).
Pour enough water into the pot just so pieces are submerged, in a 6 qt. pressure cooker 8 cups was enough.
Close the lid and steam valve. Cook on high pressure for 25 minutes.
Allow for a natural release which will take about 20 minutes. Lift lid and enjoy marrow.
Lift bones out and use a spoon to release or remove the marrow in the middle. Some may have released so you can use a slotted spoon to scoop those out. Add into a small bowl to use as a spread or otherwise.
If you want to make and save stock that is left, put a strainer inside a pot to drain the liquid. Pour liquid from pot into jars. If storing in jars make sure they cool completely first, before transferring them into the fridge.
After a few hours the fat will rise to the top, skim that off and then use as you wish up to 4 days after that.