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