Drizzle the chicken with 2 teaspoons of vegetable oil and sprinkle on the garam masala, curry powder, salt and pepper. Massage into the meat and cover, leaving to marinate for at least 2 hours or overnight. Instead of a bowl, use a 1 gallon zip top bag to marinate the chicken for a more even distribution of the marinade and easier clean up.
Heat a cast iron or grill pan over medium-high heat.
Cook the chicken for 5-6 minutes per side, until cooked to an internal temperature of at least 165 degrees F. Remove to a cutting board and cover with foil, allowing it to rest while you make the sauce.
Meanwhile, make the korma sauce. Heat 1 tablespoon vegetable oil in a large skillet or wok set over medium heat. When hot, add onions and 1/4 cup water. Sauté until onions are translucent and lightly browned, 12-15 minutes. Stir in the ground almonds and garlic and cook for 1 minute. Transfer mixture to the bowl of a blender. Add yogurt and remaining water; puree until smooth.
Measure out the spices (curry powder through nutmeg) into a small bowl.
In the same skillet or wok, heat the remaining 2 tablespoons of vegetable oil over medium heat. Once the oil is shimmering, add the spices and toast for 1 minute, then stir in the onion-yogurt puree. Cook for 2-3 minutes, stirring constantly, until it begins to darken in color.
Add the tomatoes, coconut milk, red chili, ginger, and brown sugar. Stir well.
Reduce the heat to low and simmer for 30 minutes, stirring occasionally to avoid scorching.
Cut the cooked chicken into bite-sized pieces and add to the pan. Stir in the chicken well. Simmer for an additional 15 minutes.
Serve with steamed basmati rice and/or naan.
Optionally garnish with chopped cilantro.