In a heavy saucepan, heat butter very slowly over low heat until it has melted completely without any browning. Increase heat and bring melted butter to a boil just until top is covered with foam.
Stir in onion, garlic, ginger, turmeric, cardamom, cinnamon, clove nutmeg fenugreek seeds and basil. Lower heat to lowest possible setting, and simmer, uncovered, for 45 minutes. A clear layer of liquid will be at the top of the pan, and golden brown solids on the bottom.
Pour clear liquid into a jar, straining it through several layers of cheesecloth. Discard seasonings and solids. Cover tightly and store in refrigerator, or at room temperature.