Wash and dry all produce. Halve and peel onion; finely chop one half and very thinly slice other. Core, deseed, and finely chop poblano. Finely dice tomato. Trim and thinly slice scallions. Quarter lime.

In a small, microwave-safe bowl, combine vinegar, 1 tsp sugar (2 tsp for 4 servings), a large pinch of salt, and 1 TBSP water; stir to dissolve. Stir in sliced onion. Microwave for 30 seconds; set aside to quick-pickle, stirring occasionally.

Pat chicken dry with paper towels. Heat a drizzle of oil in a large pan over medium-high heat. Add chicken and season with salt and pepper. Cook, stirring occasionally, until browned and cooked through, 4-6 minutes. Turn off heat; transfer to a plate. If necessary, wipe out pan.

While chicken cooks, in a small bowl, combine sour cream, a squeeze of lime juice, ¼ tsp chipotle powder (taste and add more if you like things spicy), and a pinch of salt and pepper. Add water 1 tsp at a time until mixture reaches a drizzling consistency. In a medium bowl, combine tomato, scallions, a large squeeze of lime juice, salt, and pepper.

Heat a drizzle of oil in pan used for chicken over medium-high heat. Add chopped onion, poblano, salt, and pepper. Cook, stirring occasionally, until veggies are softened, 3-4 minutes. Return chicken to pan and add Fajita Spice, tomato paste, stock concentrate, and ¼ cup water (⅓ cup for 4 servings). Cook, stirring, until saucy and slightly reduced, 2-3 minutes. Turn off heat. Season with salt and pepper.

Meanwhile, wrap tortillas in damp paper towels and microwave until warm and pliable, about 30 seconds. Divide chicken filling, salsa, pickled onion (draining first), and crema between tortillas. (TIP: Alternatively, put everything in individual serving bowls and let everyone assemble their own tacos.) Serve with any remaining lime wedges on the side.