Bring a large pot of heavily salted water to a rapid boil over high heat. Working in 2 batches, blanch kale for 2 minutes. Drain, let cool, and squeeze out excess water with your hands. Coarsely chop; set aside.
Heat a large pot over medium heat for 2 minutes. Add 1/4 cup oil, rosemary sprig, and chile. Let sizzle, shaking pan often, for about 1 minute. Reduce heat to medium-low; add onion. Season with 1/2 teaspoon salt and a pinch of pepper. Cook for 2 minutes, stirring often; stir in garlic. Cook, stirring often, until onion is soft and starting to brown, 5-7 minutes.
Add remaining 2 tablespoons oil and kale; stir to coat. Season with 1/4 teaspoon salt, reduce heat to medium-low, and cook, stirring often, until kale turns almost black and is slightly crisp at edges, about 30 minutes. Let cool. Discard rosemary and chile.
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