Prep time: 10 minutes
Total time: 30 minutes
Serves: 10 to 12
2 (16 oz) bags frozen small whole onions (also called pearl onions), thawed
2 Tbsp extra-virgin olive oil
3 Tbsp butter, divided
2 Tbsp balsamic vinegar
2 tsp sugar
1 tsp salt, divided, plus more for boiling beans
2 lb green beans, trimmed and cut into 1-inch to 1 1/2-inch pieces
Pat the thawed onions dry. Heat the oil in a large skillet over medium heat; add the onions and cook, stirring occasionally, until lightly browned, 6 to 8 minutes.
Stir in 1 Tbsp of the butter; when melted, add the vinegar, sugar, and 1/2 tsp salt. Cook, stirring, until the liquid is reduced to a syrupy glaze, about 5 minutes. Remove from heat.
Bring a pot of lightly salted water to a boil. Add the green beans and cook until they're bright green and crisp-tender, about 4 minutes. (They should be slightly undercooked.)
Drain and rinse under cold water to stop the cooking. Pat dry and toss with the remaining 1/2 tsp salt. Refrigerate onions and beans separately. [May be refrigerated for up to two days.]
Just before serving, melt the remaining 2 Tbsp butter in a large saucepan. Add the green beans and onions and cook over low heat, stirring occasionally, 5 to 7 minutes to heat through. (Reheating may also be done in the microwave.)