The day before...
Place the drumsticks in a large glass baking dish.
In a blender, add the pineapple and purée until smooth. Reserve half of it. And to the first half, add half the onions,the buttermilk, mustard, red chili powder and 1 tablespoon oil.
Mix well. Spread the marinade over the chicken. It should coat the pieces of meat. Plastic-wrap the dish and refrigerate overnight, turning the meat occasionally.
The next day...
Remove the chicken from the refrigerator at least 20 minutes prior cooking so it returns to room temperature. Transfer the drumsticks to a large platter, removing as much marinade as possible. Discard the marinade.
Pat the meat dry using paper towels. Season with salt and pepper.
In a wok, heat the rest of the oil. Add the remaining onion and cook on heat high until slightly golden. Lower the heat to medium-low and continue cooking until soft and tender, about 6 minutes. Transfer the mixture to a a food processor and blend until smooth. Set aside.
In the same wok, add the garlic. Cook until slightly golden. Add the cardamom and cloves. Add the chicken and sear the meat on all sides. Add the onion mixture, turmeric and reserved pineapple. Stir until the gravy become thick (about 30 seconds). Add brown sugar, basil and 2 scant cups of water. Bring to a boil, then lower the heat to medium-low. Scrape the caramelized bits from the bottom of the wok using a wooden spoon. Reduce the liquid by cooking for about 10 minutes (uncovered).
In a small bowl, dissolve the cornstarch in chicken stock. Add soy sauce and stir well.
Add the mushrooms to the chicken. Stir for 2-3 minutes, then pour in the cornstarch mixture. Bring to a boil one last time and cook until the gravy is thickened. Turn off the heat. Cover and let sit until you're ready to serve. Adjust seasoning of the gravy (if necessary) with salt and pepper. Garnish with parsley.