Combine chile paste, ginger, and garlic in a large zip-top plastic bag; add steak, tossing to coat. Marinate in refrigerator 30 minutes, turning once.
Combine lime juice, fish sauce, and sugar, stirring with a whisk; set dressing aside.
Heat a large nonstick skillet coated with cooking spray over medium-high heat. Remove steak from bag; discard marinade. Add steak to pan; cook 2 minutes or until desired degree of doneness, turning once. Cut steak into 1-inch pieces; place in a medium bowl. Add radishes, cilantro, mint, and serrano. Pour lime juice mixture over beef mixture, tossing to coat. Spoon about 1/3 cup salad into each lettuce leaf, and serve immediately.