Add the butter and oil to a 5-quart pot over medium heat. Once melted, add the onion and cook 5 minutes, stirring occasionally. Add the garlic and ginger and cook until fragrant, about 1 to 2 minutes more, stirring constantly.
4 Tablespoons unsalted butter, 1 Tablespoon vegetable oil, 1 onion, 4 cloves garlic, 1- inch piece fresh ginger
Add the garam masala, salt, red chili powder, coriander, cumin, cayenne pepper, black pepper, and fenugreek, and cook 30 seconds, stirring constantly.
1 1/2 teaspoons garam masala, 1 teaspoon salt, 1 teaspoon red chili powder, 1 teaspoon coriander, 1 teaspoon cumin, 1/2 teaspoon cayenne pepper, 1/2 teaspoon black pepper, 1/4 teaspoon fenugreek
Stir in the brown sugar and tomato sauce. Bring up to a simmer, and cook 5 minutes. Turn the heat down slightly and stir in the heavy whipping cream. Heat until it’s just very gently starting to simmer.
1 Tablespoon light brown sugar, 15 ounces plain tomato sauce, 1 cup heavy whipping cream
Carefully puree the sauce using an immersion blender or in batches using a regular blender.
Add the cooked chicken to the sauce and gently stir to combine.