Discard husks and stems from tomatillos. Combine tomatillos, cucumber, cilantro, lime juice, 1/4 teaspoon salt, and garlic in a blender. Process until finely chopped, and set aside.
Sprinkle pork with cumin. Place a large nonstick skillet coated with cooking spray over medium-high heat until hot. Add pork; cook 3 minutes or until no longer pink in center, stirring occasionally. Remove from pan; keep warm.
Coat pan with cooking spray; add chiles and onion. Coat vegetables with cooking spray; cook 4 minutes or until onion begins to brown, stirring frequently.
Meanwhile, heat tortillas according to package directions.
Return pork to pan; add remaining 1/4 teaspoon salt, and cook 30 seconds or until the pork mixture is thoroughly heated, stirring constantly.
Divide pork mixture evenly among tortillas. Top each with about 1/3 cup salsa; roll up. Serve with sour cream.