Strain the cannellini beans from the can. Rinse the beans in a sieve to wash away the excess salt. Transfer to a bowl and set aside for the time being.
Preheat your frying pan over medium heat for 2-3 minutes. Add a knob of butter.
Cut 1 onion in thick cubes. Transfer the cubes to the frying pan. Sweat the onions, stirring occasionally, till they turn gold (this typically takes about 6-7 minutes).
Mince 2 garlic cloves. Before you're done cooking the onions, sauté the minced garlic with them for 30-40 seconds, stirring often.
Transfer the beans to the frying pan. Pour in ½ of the beer from the can. Add the Dijon mustard, apple cider vinegar, and pitted prune to the pan.
Turn the heat up to medium high and cook, stirring, for 10 minutes.
2-3 minutes before you're done, sprinkle some chili flakes on your dish for an extra kick.
Notes
Optionally, add Worcestershire sauce to add some more umami flavor to this recipe. If you're a sworn carnivore, cut leftover hot dogs, beer brats, or Italian sausage crosswise into thick slices—and add them right when you add the beans and beer to the pan.