Wash the chicken and pat dry. In a baking dish large enough to hold the chicken in 1 layer, mix together the lemon juice, brown sugar, oil, garlic, paprika, garam masala, ginger, turmeric, a pinch of salt and a few grinds of black pepper. Add the chicken and toss to coat. Arrange the chicken in a single layer in the baking dish. Cover the dish with aluminum foil and transfer to the oven. Bake until the chicken reaches 150 degrees F on an instant-read thermometer, about 35 minutes.
While the chicken bakes, warm the butter in a pan over medium-low heat. Add the onions and a pinch of salt. Saute, cooking slowly, until the onions are golden brown and caramelized, about 20 minutes. Stir in the raisins and rum, and use a wooden spoon to loosen any onion bits from the bottom of the pan. Cook until the alcohol evaporates, leaving just the rum scented flavor, about 2 minutes, then remove the pan from the heat. When the chicken reaches 150 degrees F, spoon the onion-raisin mixture over the chicken. Raise the oven temperature to 400 degrees F, then return chicken to the oven and bake for 10 minutes more, uncovered. The chicken will be fully cooked when it reaches 160 degrees F. Transfer to a serving platter and serve warm or cold.