Process garlic, onion, and ginger in a food processor until very finely chopped, about 15 seconds, stopping to scrape down sides of bowl as needed.
Melt butter in a small saucepan over medium, stirring occasionally.
Add onion mixture, coriander seeds, fenugreek seeds, cardamom, cumin, turmeric, and nutmeg.
Reduce heat to medium-low; cook, stirring occasionally, until white foam dissipates and butter is a deep golden color, 6 to 8 minutes.
Remove from heat.
Let butter mixture infuse for 1 hour.
Pour butter mixture through a fine wire-mesh strainer lined with a cheesecloth and placed over a small bowl; discard solids.
