Rinse the drained black beans in a sieve with cold water. Use a hand blender to chop until only small pieces remain, or mash thoroughly with a fork if you don't have a hand blender.
Peel and finely grate the large onion. Press the large garlic clove.
In a bowl, mix together the chopped beans, grated onion, pressed garlic, egg, oatmeal, mustard, salt, vegetable stock powder, pul beaver, marjoram, and pepper. Gradually add breadcrumbs until the mixture becomes firm and easily moldable.
Refrigerate the mixture for 1 hour to firm it up and help it hold together better.
Heat the oil in a large pan over medium heat. Shape the bean mixture into 6 patties and fry until golden brown on both sides.