In a large skillet on Wolf cooktop over medium heat, melt butter. Add onions and sauté 4 minutes or until transparent. Add pistachios and pepper flakes. Continue to sauté 3 minutes. Toss with sprouts and cook 3 minutes. Add lemon juice and continue to cook about 3 minutes more or until sprouts are bright green and slightly tender. Season with salt. Sprinkle cheese, if desired, over all immediately before serving.