Mix together, in a small bowl, salsa, tomato paste, olive oil, garlic, lime juice, pepper, and salt.
Place flank steak into the bottom of the slow cooker then pour mixture on top. Lay the onion and peppers on top.
Cook for 5 – 8 hours on low. **I used a 6 quart slow cooker and put mine in frozen. It was done at 6 hours. If I put it in thawed it would have been done probably at around 5 hours (or less). Depending on your slow cooker your time could vary.
Remove steak from slow cooker and cut across the grain into thin slices.
Mix together avocado, greek yogurt, salt, pepper, and lime.
Divide steak, peppers and onions, and avocado spread among 6 tortillas. Roll up and serve warm.