Quick-Pickle the Red Onions: Heat the vinegar, water, sugar, and salt until the sugar dissolves. Pour over the sliced red onion in a jar and let sit at least 30 minutes (they keep for 2 weeks in the fridge). These tangy onions cut through the creamy chicken beautifully.

Build the Butter Chicken Sauce: Melt 2 tablespoons of butter with the oil in a large, heavy skillet or Dutch oven over medium heat. Add the onion and cook 6–8 minutes until soft and golden. Stir in the ginger garlic paste and cook 1 minute until fragrant. Add the crushed tomatoes, drained cashews, Kashmiri red chili powder, turmeric, garam masala, and salt. Pour in the water, bring to a simmer, cover, and cook 12–15 minutes until thick and deep orange-red.

Blend Until Silky: Let the sauce cool slightly, then blend until completely smooth with an immersion blender (or in a regular blender, venting the lid). For the smoothest sauce — the kind that clings to shredded chicken — strain it through a fine-mesh sieve back into the pan.

Simmer and Shred the Chicken: Return the sauce to medium-low heat. Stir in the cream, crushed kasuri methi, and the remaining 1 tablespoon of butter. Nestle the chicken thighs into the sauce, cover, and simmer gently for 25–30 minutes, turning once, until the thighs are fork-tender (165°F / 74°C internal temperature). Transfer the chicken to a plate and shred it with two forks, then fold the shredded chicken back into the sauce. Simmer 5 more minutes so the shreds soak everything up. Slow cooker variation: Add all the sauce ingredients (steps 2–3, blended and strained) plus the chicken thighs to the slow cooker. Cook on LOW for 4–5 hours or HIGH for 2½–3 hours. Shred the chicken, stir in the cream and kasuri methi, and cook 15 more minutes on HIGH.

Make the Coleslaw: Toss the coleslaw mix with the mayonnaise, lime juice, and a pinch of salt. The crunch and acidity balance the rich, creamy chicken.

Assemble the Sandwiches: Butter the cut sides of the brioche buns and toast them in a skillet until golden. Pile each bottom bun high with shredded butter chicken, top with a scoop of coleslaw, a few pickled red onions, and fresh cilantro. Cap with the top bun and serve immediately.