Cook 5 sliced garlic cloves in 1/4 cup olive oil in a pot over medium-low heat until sizzling, 1 minute. Add 1 pound chopped kale and stir until wilted, 3 to 5 minutes. Increase the heat to medium. Add 2 tablespoons cider vinegar, 1/2 teaspoon kosher salt and a pinch of red pepper flakes. Cook, stirring, until tender, 5 minutes.