Prick the sweet potatoes a few times with a fork. Microwave, turning halfway through, until almost tender, about 8 minutes. Let cool slightly, then cut into bite-size pieces.
Heat 2 tablespoons vegetable oil in a large nonstick skillet over medium-high heat. Add the shishito peppers and season with salt and pepper. Cook, turning occasionally, until browned and starting to soften, 2 to 4 minutes. Remove to a plate with tongs. Add the sweet potatoes to the skillet; season with salt and pepper. Cook, stirring occasionally, until browned and tender, 6 to 8 minutes. Return the peppers to the skillet to heat through, about 30 seconds.
Meanwhile, mix the soy sauce, vinegar and 1 tablespoon water in a small bowl. Season the steaks all over with salt and pepper. Heat the remaining 1 tablespoon vegetable oil in a separate large skillet over medium-high heat. Add the steaks; cook until browned, 4 minutes per side for medium rare. Remove to a cutting board.
Place the empty skillet over medium heat and add 1 tablespoon butter and the ginger. Cook until the ginger sizzles, about 30 seconds. Remove from the heat and add the soy sauce mixture. Whisk in the remaining 3 tablespoons butter, 1 tablespoon at a time, until smooth.
Slice the steaks and divide among plates, along with the vegetables. Spoon the sauce over the meat. Top with sesame seeds.
Leave the skin on your sweet potatoes! You’ll get even more beta-carotene, which may help prevent skin damage.