Wash and dry all produce. • Halve and peel onion; very thinly slice one half and finely chop remaining onion. Core, deseed, and dice poblano into ½-inch pieces. Roughly chop cilantro. Dice tomato.
In a medium bowl, combine sliced onion, vinegar, 2 tsp sugar, salt (we used ½ tsp), and 1 TBSP water. Stir until sugar and salt are mostly dissolved. Set aside, stirring occasionally, until ready to serve.
Heat a drizzle of olive oil in a large pan over medium-high heat. Add pork* and season with salt and pepper. Cook, breaking up meat into pieces, until browned and cooked through, 4-6 minutes. • Transfer to a plate and set aside.
Heat another drizzle of olive oil in same pan over medium-high heat. Add poblano and chopped onion; season with salt and pepper. Cook, stirring, until softened, 3-4 minutes. • Return pork to pan along with Southwest Spice, stock concentrate, half the tomato paste (all for 4 servings), and ¼ cup water (⅓ cup for 4). Cook, scraping up any browned bits from bottom of pan, until mixture is thickened and saucy, 1-2 minutes more.
While filling cooks, in a small bowl, combine sour cream, a bit of onion pickling liquid to taste (start with ¼ tsp and add more if desired), and a pinch of chipotle powder (add more to taste if you like things spicy). Stir in water 1 tsp at a time until mixture reaches a drizzling consistency. Season with salt and pepper. • Wrap tortillas in damp paper towels and microwave until warm and pliable, 30 seconds.
Divide tortillas between plates; fill with pork filling, tomato, Monterey Jack, cilantro, pickled onion (draining first), and crema. TIP: Alternatively, you can put everything in individual serving bowls and let everyone assemble their own tacos.
