Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels, reserving 1 tablespoon bacon grease in skillet.
Cook and stir onion in the bacon grease over medium heat until softened, 5 to 10 minutes. Add kale; cook and stir until slightly wilted, 2 to 3 minutes. Stir garlic into kale mixture and cook until garlic is fragrant, about 1 minute.
Transfer kale mixture to a serving bowl; crumble bacon, sprinkle cashews, and drizzle rice vinegar over kale.