Bring a large pot of salted water to a boil. Blanch the kale for 1 ½ -2 minutes. Drain, let it cool until it's easy to handle, and squeeze out excess water. Chop and set aside.
In a large skillet, warm ¼ cup of the olive oil over medium heat. Add the rosemary sprigs and chile and cook for one minute.
Reduce the heat to low, add the onion and a strong pinch of salt; cook for a couple more minutes. Add the garlic slices and stir. Cook until the onions are soft and starting to brown (~ 8-10 minutes)
Turn the heat to medium-low, add the kale and add the last couple of tablespoons of oil; stir to coat. Cook for 25-30 minutes, stirring occasionally until the kale is almost black and charred at the edges.
Remove the rosemary sprigs and the chile de Arbol, if using. Top with a few grinds of freshly ground pepper, and taste for salt.