Adjust rack to top position (top and middle positions for 8 servings) and preheat oven to 425 degrees. Wash and dry produce. Halve, peel, and thinly slice onion; finely dice some of the slices until you have 1/4 cup (1/2 cup for 8 servings). Roughly chop cilantro, including tender stems. Dice tomatoes into 1/2-inch pieces. Quarter lime. Halve, deseed, and slice green pepper into strips. Peel and finely chop garlic.

Add diced onion, cilantro, half the tomatoes, and juice from lime wedges to a small bowl. Season with salt and pepper; stir to combine.

In a medium microwave-safe bowl, whisk together cornstarch, half the Tex-Mex paste, one stock concentrate, and 1 cup hot water until smooth (two stock concentrates and 2 cups hot water for 8 servings). Cover with a paper towel and microwave 1 minute. Uncover; whisk until smooth. Cover and microwave until thickened, 30 seconds more. Whisk until smooth. (The sauce should be the consistency of gravy—if it isn’t, continue microwaving and whisking in 30-second intervals.)

Pat chicken dry with paper towels. Heat a drizzle of oil in a large pan over medium-high heat. Add green pepper and sliced onion to hot pan; cook, stirring occasionally, until beginning to soften, 1-2 minutes. Add chicken and garlic; cook, stirring occasionally, until chicken is browned and cooked through, 4-6 minutes. Add Southwest Spice Blend, remaining tomatoes, remaining stock concentrates, and remaining Tex-Mex paste to pan; stir to coat. Turn off heat.

Place a small amount of filling on bottom half of each tortilla. Roll up tortillas, starting with filled sides, to form enchiladas. Place, seam sides down, in a 9-by-13-inch baking dish (use 2 baking dishes for 8 servings). Pour Tex-Mex sauce over enchiladas to thoroughly coat. Sprinkle with Mexican cheese blend and Monterey Jack. Bake on top rack (top and middle racks for 8) until sauce is bubbling and cheese melts, 3-5 minutes.

Top enchiladas with pico de gallo and dollop with sour cream. Divide between plates or serve family style directly from baking dish. Chicken is fully cooked when internal temperature reaches 165°. When reheating leftovers, be sure they reach 165° as measured with a food thermometer.