Heat 2 tablespoons of the oil in a large skillet over medium heat. Place the onions in skillet and cook for 5 minutes. Add the jalapeños, garlic, ginger, and 4 tablespoons of the lemon juice and cook for 5 minutes more. Transfer the mixture to a plate. Add the remaining 2 tablespoons of oil to skillet and raise heat to medium-high. Add the chicken and stir-fry until golden, about 8 minutes. Add the cayenne, coriander, cumin, mustard seeds, and salt and stir-fry for 2 minutes more. Add the peaches, onion mixture, and 1/2 cup water and simmer for 8 minutes. Stir in the coconut milk, reduce heat to medium-low, and simmer gently for 2 minutes. Taste and add the remaining lemon juice, if necessary. Serve with the yogurt, flat bread, or both, if desired.