Wash and dry all produce. Adjust oven rack to upper position and preheat oven to 450 degrees. Halve, peel, and dice onion. Trim, then thinly slice scallions, separating greens and whites. Halve, core, and seed poblano, then cut into 1/2-inch pieces. Pat chicken dry with a paper towel.

Heat a drizzle of oil in a large pan over medium-high heat. Add onion and poblano and cook, tossing, until slightly softened, about 2 minutes.

Add chicken and half the Southwest spice to same pan. Cook, tossing occasionally, until chicken is browned and cooked through, 4-5 minutes. Season with salt and pepper. Remove pan from heat and set aside.

Heat a drizzle of oil and 1 TBSP butter in another, medium-sized pan over medium-high heat. Add scallion whites, enchilada spice, and remaining Southwest spice. Cook, stirring, until fragrant, 1-2 minutes. Add 6 TBSP tomato paste (we sent more) and cook 1-2 minutes, stirring constantly. Stir in 1 cup water and 1/2 tsp sugar. Bring to a simmer and let thicken slightly, about 2 minutes. Season with plenty of salt and pepper. Remove pan from heat.

Place a tortilla on your work surface and add half the chicken mixture towards one side. Roll up tortilla and place seam-side down in a small baking dish or ovenproof pan. Repeat with remaining tortilla and chicken mixture, placing rolls side-by-side. Spoon over enough sauce to generously coat (you may not need all). Sprinkle evenly with cheese.

Place dish or pan in oven and bake until sauce is bubbly and cheese melts, about 2 minutes. Dollop baked enchiladas with sour cream. Sprinkle with scallion greens. Divide between plates and spoon over any remaining sauce.