Pour 1/3 the jar of salsa into a slow cooker.
Place the chicken in the slow cooker, and top with remaining salsa.
Cover and cook on high for 3-4 hours, or low for 6 hours, or until the chicken falls apart and shreds easily with a fork. Shred the chicken thoroughly, and stir with the juices. Turn off the slow cooker.
While chicken is cooking, prepare remaining ingredients.
In a large saucepan over medium heat, melt the butter.
Add the onion and peppers, cooking and stirring occasionally, until they begin to brown and soften, about 10-15 minutes.
Layer the flour tortillas with the salsa chicken, onions and peppers, avocado, a dollop of Greek yogurt, and a generous sprinkling of cheese.
Serve immediately.