Step 1

Heat oil in large skillet over medium heat. Add onion and cook until crisp-tender and just beginning to brown, about 5 minutes. Stir in the salsa, tomatoes and vinegar. Simmer, stirring regularly until quite thick, about 5 minutes. Stir in the chicken, then taste and season with salt.

Step 2

Serve the mixture in warm tortillas. Let guests add avocado, cheese and cilantro to taste.