I've developed this over the years from some slow cooker carne asada recipe; but this is not carne asada, so I have renamed it for what it is: slow-cooked beef in chili spices. Also I will tell you what I have learned from years of trying to make everything in a slow cooker: 1) Don't try to cook meat and vegetables together in a slow cooker, not to eat together; vegetables that you cook with meat in a slow cooker are for seasoning, not eating. 2) Don't slow cook meat in liquid (e.g., broth), it just makes the meat watery-tasting and weirdly also dry-tasting. Just vegetables for seasoning will exude enough (sometimes too much) liquid for cooking meat in.
a red bell pepper
a jalapeno pepper
2 Tbsp chili powder
1 Tbsp cumin
1/4 tsp cayenne pepper
2 lb flank steak
Roughly chop the onion and peppers and mix together with all the spices in the slow cooker. Cut the meat crosswise into roughly two-inch strips. Layer the meat in the aromatics and spices in the slow cooker and cook on low until falling apart, about four hours but maybe longer. Fish the meat out of the vegetables, pull it apart into bite-size pieces, and put it into a container to put in the fridge.
You can strain the vegetables from the meat juices, or puree the vegetables into the meat juices, to pour over the meat in the container.
The nicest way to serve this is to scoop however much meat you want to eat in a singleish layer on a baking sheet and broil for ten minutes to char, then stir it up and serve.
The easiest way is to just scoop it right into your bowl and reheat in the microwave, it's still good.