Bring a large pot of salted water to a boil. Wash and dry produce.

Halve, peel, and thinly slice onion. Dice tomato into ¼-inch pieces. Finely chop cilantro. Quarter lime. 

Once water is boiling, add chicken* to pot; poach until cooked through, 10-15 minutes.

Transfer chicken to a large bowl and shred meat with two forks. Discard poaching liquid.

While chicken poaches, heat a drizzle of oil in a large pan over medium-high heat. Add onion, 1 TBSP sugar (2 TBSP for 4 servings), salt, and pepper; cook, stirring occasionally, until onion is softened and transluscent, 6-8 minutes.

Transfer onion to a plate.

Add a drizzle of oil to pan used for onion over medium-high heat. Add tomato paste and cook, stirring, until darkened and fragrant, 30-60 seconds. Stir in sofrito, stock concentrate, Mexican Spice Blend, and ⅓ cup water (⅔ cup for 4 servings). Cook, stirring occassionally, until liquid has reduced and slightly thickened, 1-2 minutes.

Add caramelized onion and shedded chicken to pan with sauce; stir until fully combined. 

Wrap tortillas in damp paper towels and microwave until warm and pliable, 30-60 seconds.