Whisk miso, mirin and rice vinegar together in a small bowl. Whisk in garlic. Pat tilapia fillets dry with paper towel and arrange on a plate. Spoon miso marinade over fillets, making sure to coat fish completely on both sides. The marinade will be fairly thick and cling nicely to the fish. Let fish marinate on kitchen counter for 30 minutes. Meanwhile, toast the sesame seeds in a dry skillet over medium-low heat until golden, stirring occasionally to avoid burning, about 3 to 4 minutes. Transfer to a bowl and set aside. Prepare the scallions. Trim just the root tips and slice the scallions lengthwise, using the tip of a very sharp knife. When fish has finished marinating, gently wipe off excess marinade (I used my fingers), but do not rinse. Season on both sides with chili pepper and freshly ground pepper. Heat two tablespoons of olive oil in a large nonstick skillet over medium flame until shimmering. Sauté fillets until just cooked through, 3 to 4 minutes per side, turning once. Transfer to serving platter or individual plates. Add scallions to skillet, gently tossing to coat with oil. Cook until they just wilt, 30 seconds to a minute. Arrange on fillets and serve. A confessional note about the scallions: Prepared this way, they make for an elegant presentation, but cutting the barely cooked scallions while eating is less than easy. For simpler dining, halve them lengthwise as described above, then slice them crosswise into 2- to 3-inch pieces before cooking. So I’ll leave it to you—you can go for the wow factor or easier eating.