Sauté Onions and Aromatics: In the same skillet, add the butter and vegetable oil. Once the butter has melted, add the finely chopped onions and sauté until they turn golden brown.
Add Garlic, Ginger, and Chilies: Add the minced garlic, grated ginger, and slit green chilies. Sauté for another 2-3 minutes until fragrant.
Add Tomato Purée and Spices: Stir in the tomato purée, ground cumin, ground coriander, turmeric powder, chili powder, and salt. Cook the mixture on medium heat, stirring occasionally, until the oil separates from the tomato mixture and it thickens (about 10-15 minutes).
Finish the Sauce: Reduce the heat and stir in the heavy cream. Add the garam masala and crushed fenugreek leaves (kasuri methi). Simmer for another 5 minutes, allowing the flavors to meld together.