Bring 12 cups water, raw peanuts, ginger, serrano half, garlic, lemon juice, 2 tablespoons salt, and turmeric to a boil in a Dutch oven over high. Reduce heat to medium, cover, and gently boil until peanuts are tender, but not mushy, about 3 hours and 30 minutes. Drain peanut mixture; discard ginger, serrano half, and garlic. Place peanuts in a large bowl; set aside to let cool.
Meanwhile, heat oil in a large skillet over medium. Add cumin seeds and curry leaves; stir until seeds and leaves crackle and become very fragrant, about 1 minute. Add dry-roasted peanuts; toss to coat, and remove from heat.
Add roasted peanut mixture, onion, tomatoes, cucumber, cilantro, lime juice, mint, chopped serrano, chaat masala, honey, cayenne, and remaining 1/2 teaspoon salt to boiled peanuts; toss to coat. Serve immediately.