Puffy Chicken Tacos

Name: Puffy Chicken Tacos
Ingredients: 1⁄4 cup olive oil; 2 1⁄2 lb. bone-in, skin-on chicken thighs; 6 cloves garlic, finely chopped, plus 2 cloves minced; 1 medium yellow onion, finely chopped; 1 rib celery, finely chopped; 1 small carrot, finely chopped; 1 small red bell pepper, finely chopped; 3 cups chicken stock; 1 (14 oz.) can whole peeled tomatoes in juice, crushed by hand; 2 avocados, pitted, peeled, and mashed; Juice of 1 lime; Canola oil, for frying; 3 cups masa harina (available from amazon.com); 1 1⁄2 tbsp. unsalted butter, softened; Shredded iceberg lettuce, diced tomato, and shredded cheddar cheese, to garnish; Kosher salt and freshly ground black pepper, to taste