Heat a grill pan over high heat. (Or get your outdoor grill heated up to high/medium-high.) Sprinkle steak evenly with salt and pepper. Lightly coat the steak with cooking spray. Add steak to pan; grill for 5 minutes on each side or until desired degree of doneness. Let stand 3 minutes.
Heat a small skillet over medium-high heat. Add oil to pan; swirl to coat. Add garlic; cook for 30 seconds or until fragrant. Add soy sauce, mustard, and sugar; cook 1 minute or until bubbly. Remove pan from heat. Stir in cream and 1 1/2 tablespoon cilantro.
Cut steak diagonally across grain into thin slices. Sprinkle with remaining 1 1/2 tablespoon cilantro. Serve sauce with steak.