In a medium bowl, combine the chicken thigh pieces with yogurt, 1 tbsp garlic, 1 tbsp ginger, turmeric, garam masala, and salt. Mix well to coat. Cover and refrigerate for at least 30 minutes or up to 6 hours.

Melt butter in a large pot or Dutch oven over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the remaining 1 tbsp garlic and 1 tbsp ginger, and cook for another minute until fragrant.

Stir in the ground cumin, coriander, and cayenne pepper. Cook for 1 minute until aromatic. Pour in the crushed tomatoes and add the soaked cashews. Bring to a simmer, then reduce heat to low, cover, and let it cook for 15 minutes, stirring occasionally.

While the sauce simmers, heat the canola oil in a large skillet over medium-high heat. Add the marinated chicken in a single layer (work in batches if necessary). Cook for 3-4 minutes per side, until golden brown and cooked through. Remove and set aside.

Once the sauce has simmered, use an immersion blender to blend it until smooth and creamy. Alternatively, carefully transfer it to a traditional blender and blend until smooth.

Return the blended sauce to the pot. Stir in the cooked chicken. Reduce the heat to low and pour in the heavy cream. Crush the dried fenugreek leaves between your palms and stir them into the sauce. Simmer gently for 5 more minutes. Do not boil. Serve hot, garnished with fresh cilantro.