Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 2-quart baking dish.
Heat oil and butter in a skillet over medium heat. Cook onions in oil until soft.
Place onion, yogurt, mango chutney, garlic, turmeric, chili powder, garam masala, and into the bowl of a food processor. Process into a smooth sauce; it should be about the consistency of thick cream. Add a bit of water or yogurt to thin it if needed. Spread chicken into prepared baking dish, and pour the onion sauce over the chicken.
Bake in preheated oven for 30 minutes, or until the chicken is cooked through. Sprinkle sliced almonds over the top, and serve.