For dressing, in a small bowl combine basil, red pepper, lime juice, 3 Tbsp. warm water, fish sauce, rice vinegar, and sugar. Stir until sugar dissolves. Set aside. Sprinkle both sides of steaks with salt.
Heat a 14-inch flat-bottomed wok or 12-inch stainless-steel skillet over medium-high heat until a bead of water vaporizes within 2 seconds of contact. Swirl in 1 Tbsp. of peanut oil. Add steaks; pan-fry 3 to 5 minutes or until medium-rare, turning once halfway through cooking. Remove steaks to a platter; set aside.
Swirl remaining 1 Tbsp. peanut oil into pan over high heat. Add green onions; stir-fry 30 seconds or until slightly softened. Add zucchini and yellow squash; stir-fry 1 minute. Transfer to bowl. Add spinach to pan; cook 15 seconds. Stir into vegetables. Stir in basil leaves. To serve, spoon vegetables over steaks. Drizzle with dressing.