To Marinate the Chicken: In a small skillet, toast fenugreek leaves (or fenugreek seeds, if using) over medium heat, tossing them constantly, until quite fragrant, about 30 seconds. Transfer toasted leaves to spice grinder or mortar and pestle and grind to fine powder. Set aside.

In the center of a rimmed baking sheet, create a roughly 9- by 13-inch aluminum-foil boat with 1-inch sides, and set aside. In a medium mixing bowl, stir together yogurt, garam masala, salt, black salt, grated ginger, and ground fenugreek leaves.

Add chicken thigh pieces to bowl and, using clean hands, toss with marinade until evenly coated. Transfer chicken to prepared baking sheet, arranging pieces in a single, even layer in the aluminum-foil boat.

For the Makhani Sauce: Add fenugreek leaves (or seeds, if using), chiles de arbol, brown cardamon (or green cardamom, if using), and clove to small skillet and place it over medium heat. Toast, tossing frequently, until spices are quite fragrant, about 1 to 2 minutes. Transfer spices to spice grinder or mortar and pestle along with garam masala and salt and grind to a fine powder. Set aside.

In a small, microwave-safe bowl, combine cashew nuts and ¼ cup (60ml) water. Microwave on high until cashews look plump and have softened slightly, about 1 minute. Set aside.

In a Dutch oven, heat canola oil over medium-high heat until shimmering. Add onions and baking soda and, using a wooden spoon, stir to coat onions in oil and distribute baking soda. Cook, stirring occasionally, until onions have completely broken down, most of their moisture has cooked off, and they begin to brown, 14 to 17 minutes.

Reduce heat to medium low. Add ginger and garlic to pot and cook, stirring constantly, until quite fragrant, about 1 minute. Using a wooden spoon, push onions into center of pot to form a mound. Add ground spice mixture to the mounded onions to prevent spices from scorching. Cook, stirring constantly, until onions are coated in spices and mixture is very fragrant, about 30 seconds.

Add cashews and their soaking liquid, scraping up any bits stuck to the bottom of the pot. Add tomatoes and their juices plus 1 cup (240ml) water and, using the back of wooden spoon, crush tomatoes.

Bring to a boil, then reduce heat to maintain gentle simmer. Cook, stirring occasionally, until tomatoes are completely broken down and liquid has reduced, about 40 minutes.

Meanwhile, adjust oven rack to about 3 inches below broiler element and preheat broiler on high. Transfer chicken to broiler. Cook, checking the chicken frequently to ensure it's not burning, until chicken is charred in spots and is fully cooked through, about 14 minutes.

Using an immersion blender and off the heat, blend contents of Dutch oven until completely smooth, about 2 minutes. Add butter and cream, and blend until completely smooth and emulsified, about 2 minutes.

To Finish: Add reserved broiled chicken along with any juices in the sheet pan to sauce and stir until chicken is well incorporated and warmed through. Ladle chicken and sauce into serving bowl and drizzle with additional heavy cream. Serve immediately with rice alongside.