Braised Kale With Caramelized Onions

8 cups torn and stemmed kale pieces, firmly packed
1 Tbsp. olive oil
2 cups thinly sliced onions
1 tsp. salt
1/2 tsp. ground black pepper
1/2 tsp. red pepper flakes
2 Tbsp. minced garlic
2 cups vegetable stock
1 tsp. cider vinegar

•In 2 batches, blanch the kale for 2 minutes in a 6- to 8-quart pot of boiling salted water. Transfer with tongs to a colander and drain well.

•Heat the oil in a large skillet over medium heat. Add the onions, salt, pepper, and red pepper flakes and cook until the onions are golden brown, stirring frequently.

•Add the garlic, kale, and stock and cook, stirring occasionally, for 8 to 10 minutes, or until tender.

•Add the cider vinegar in the last minute of cooking, then remove from the heat. Serve immediately.