Tom Valenti's Sautéed Kale with Garlic and Red Onions

In a large skillet over medium-high heat, heat 2 tablespoons olive oil. Add onions and cook until soft, about 8 minutes. Transfer to a bowl. Reduce heat to medium-low, add garlic, and cook for 2 to 3 minutes. Transfer to bowl with onions. Wipe any blackened bits from skillet and add remaining oil. Increase heat to medium-high and add kale; cook, stirring occasionally, until kale wilts, about 5 minutes. Add reserved onion and garlic. Season with 1/2 teaspoon each salt and pepper. Reduce heat to medium-low and continue cooking until kale is tender, about 5 minutes. Sprinkle with vinegar and toss.