Cut flank steak in half lengthwise, then crosswise into thin strips. Place into slow cooker.
Combine salsa, onion, cilantro, lime juice, garlic, chili powder, cumin, and salt. Pour over meat and cook on high for 4 hours or on low for 5-6 hours.
Add red bell pepper and cook for 1 more hours on low. Serve with warmed tortillas.
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