For pickled onions
Bring rice wine vinegar to a boil in a small saucepan. Add sugar and salt, and stir until dissolved. Turn off the heat and let mixture cool for about 5 minutes. Put diced onion in a large heatproof container, and pour vinegar mixture over. When mixture is room temperature, refrigerate for at least a day.

For brussels sprouts
Preheat overn to 425 degrees. Cut pastrami into strips. Melt butter in a large skillet or saute pan over medium heat. Add pastrami and cook over medium heat until browned fat has rendered out, about 3 minutes. Pour pastrami and fat into a large heatproof mixing bowl. Add Brussels sprouts, salt and pepper, and toss to combine. Line two large baking sheets with foil and spread Brussels sprouts out on them in a single layer. Don''t crowd the pans. Roast for 35 minutes, removing pans from the oven at the 20-minute mark to move the Brussels sprouts around. Brussels sprouts will be dark brown and crispy. Strain the pickled red onions and reserve 1/2 cup of pickling liquid. As soon as the Brussles sprouts come out of the oven, toss them with the red onions and the 1/2 cup of liquid while they are still warm. Serve immediately.