Preheat an outdoor grill for high heat and lightly oil the grate.
Blend soy sauce, vinegar, brown sugar, sesame oil, garlic, and ginger together in a blender or food processor until dressing is smooth.
Press peppercorns into both sides of steak and season with salt.
Place steak, scallions, and red bell pepper on the preheated grill and cook for 4 minutes. Flip and cook until vegetables are charred and steak is cooked to desired doneness, about 4 minutes more. Let steak rest on a work surface for 5 minutes before thinly slicing along the grain. Cut scallions into 1-inch pieces and bell pepper lengthwise into strips.
Toss curly endive and radicchio with the dressing in a bowl; transfer to a platter and top with steak and vegetables.