Combine miso paste, Dijon mustard, rice vinegar, sesame oil, soy sauce, and chili flakes in a blender; blend on low speed until combined. Increase the blender speed to medium-high, and slowly stream in ¼ cup of oil. Set aside.*You can also whisk this by hand if you don't want to dirty the blender.
Preheat oven broiler to HIGH.Heat remaining 2 Tbsp. of oil in a large oven-safe skillet over medium-high. Once hot, add Brussels sprouts and cook for 7 to 8 minutes, stirring every few minutes, until nicely browned and tender. Season with salt. Meanwhile, add panko, Parmesan, and sesame seeds to a medium bowl; toss to combine.
Pour miso-sesame mixture into skillet with Brussels sprouts and toss to coat. Sprinkle panko mixture overtop, and transfer skillet to upper center rack of oven. Broil for ~2 minutes, until a golden-brown crust forms. *Watch closely to prevent burning.Remove from oven, stir, and serve!
