Put the flank steak into the slow cooker and add in the rotel, garlic, onion, and fajita seasoning. If you want to mix, give everything a quick stir.
For a quicker dinner, you can turn the crock pot to high and cook for four hours. For a slower cook, put the slow cooker to the low setting and cook for eight hours. During the last hour, add in the sliced peppers and toss with the extra liquid and spices. (This would be done at hour three if cooking on high and hour seven if cooking on low.)
Once the cooking time is complete, take the meat out of the crockpot and shred it. The meat should be extremely tender and shred easily. Put the meat back into the crock pot and stir with the vegetables. (If the meat isn't tender, put it back into the crockpot and cook for 30 more minutes up to an hour to help it tenderize.)
Use tongs and serve the shredded steak and veggies. (a slotted spoon works great, too!) Make sure to remove any liquid before adding to the tortilla. Add optional toppings and enjoy!