Layer half of the onions and peppers in the bottom of your slow cooker. Top with the flank steak. (For easier shredding later and shorter, less stringy pieces, I like to cut the flank steak into four or five pieces, crosswise, before placing it in the slow cooker.)
In a small bowl, mix together the chili powder, cumin, oregano, and salt. Evenly sprinkle over the flank steak, flipping the meat to get some of the spice mixture on both sides.
Top with the remaining onions, peppers, and the minced jalapeño.
Cook on low for 7 to 8 hours (or on high for 3 to 4 hours) or until the flank steak can be easily shredded with two forks. For ease, remove the steak to a dinner plate to shred.
At this point, you may place the meat back in the slow cooker for up to another hour or serve immediately, mixed with a little of the juice as desired.
Heat tortillas, if using, or serve over rice. Top as desired and enjoy!