Oxtail Guisado

Name: Oxtail Guisado
Ingredients: 2½ lb. ½–¾ thick oxtails; 1 large red onion, thinly sliced; 1 large green bell pepper, halved, ribs and seeds removed, cut into strips; 1 bunch cilantro, tough stems removed, cut into 2-long pieces; 6 garlic cloves, finely grated; 1 1.4-oz. packet sazón con culantro y achiote (such as Goya; about 2 tsp.); 2 Tbsp. adobo seasoning (such as Goya); 2 Tbsp. dried oregano; 2 Tbsp. extra-virgin olive oil; 3 oz. tomato paste (about 6 Tbsp.); 2 avocados, cut into about ¾ pieces; 1 large cucumber, peeled, cut into about ¾ pieces; Juice of 1 lime; ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, plus more; Freshly ground pepper