In a blender, pulse 4 tomatoes until well chopped. Add 1 1/2 tsp salt, jalapenos, 1/4 white onion, and garlic and puree until smooth.
2
Heat oil in skillet over medium-high heat. Brown chicken, about two minutes per side, working in batches if necessary. Add tomato puree to skillet. Cover and reduce heat to medium low.
3
Simmer chicken 30 minutes, or until very tender, stirring occasionally. Shred into bite-size pieces using two forks. Increase heat to medium and cook, stirring often, until sauce thickens and coats chicken, about 10 minutes. Stir in cilantro just before serving.
4
Make avocado salsa. Cut two avocados into 3/4 inch cubes. Mix avocado with 3 Tbsp diced red onion, 1 seeded and diced roma tomato, juice of one lime, and 1/2 tsp kosher salt.
5
Serve chicken in corn tortillas with avocado salsa.