Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add Brussels sprouts, cover, and steam until tender yet crisp, 8 to 10 minutes.

Melt butter in a deep skillet over medium-high heat. Cook and stir onions and 3 tablespoons vinegar in hot butter until onions brown.

Add steamed Brussels sprouts, sugar, and remaining 1 tablespoon vinegar to onions in the skillet. Sauté over medium heat until Brussels sprouts are lightly caramelized.

Season with salt and pepper. Garnish with pistachios to serve.