Heat olive oil in a large saucepan over medium-high heat.
Add crushed red pepper flakes, if using, and let them sizzle a bit in the oil.
Add the garlic and cook until soft, but not colored.
Raise heat to high, add the stock and kale and toss to combine.
Cover and cook for 5 minutes.
Remove cover and continue to cook, stirring until all (most) the liquid has evaporated. I like the kale to still remain bright-ish green.
Season with salt and pepper to taste and add vinegar.