2.

Heat oil or ghee in a large saucepan or Dutch oven over medium-high heat until shimmering. All at once, add mustard seed and cumin. They will sputter and spit for a few seconds. As soon as they are aromatic (about 15 seconds), add onion all at once, along with baking soda. Cook, stirring frequently, until onions start to leave a brown coating on bottom of pan, 3 to 4 minutes. Add 1 tablespoon water, scrape up browned bits from pan, and continue cooking. Repeat this process until onions are a deep brown, about 10 minutes total.