Methods

1. Using the tip of a spoon, peel the turmeric, gently scraping it against the outside of the root to remove the skin (using a spoon will help you get any nooks, crannies, and curves without sacrificing too much of the turmeric itself). Discard the peel, taking one piece of peeled turmeric at a time, thinly slice it lengthwise into ¼” slices. Then, slice lengthwise into ¼” matchsticks and cut crosswise into a ¼” dice. Then, husk the tomatillos and slice into ¼” slices, then ¼” matchsticks, and a ¼” dice. Keep the turmeric and tomatillos separate and set aside.
2. Heat the mustard seed oil in a medium saucepan over medium heat. When the oil is shimmering, add the cumin seeds, coriander seeds, fennel seeds, black mustard seeds, nigella seeds, and asafoetida, if using, and toast the spices, stirring frequently, until fragrant and the coriander seeds turn a couple shades darker, 30 seconds to 2 minutes. 
3. Stir in the reserved diced fresh turmeric and ¼ teaspoon salt, and cook, stirring occasionally, until the turmeric just starts to soften, 2 to 4 minutes. Add the Byadgi chili powder and 3 tablespoons water, and cook, stirring occasionally, until most of the water has evaporated and the oil starts to separate, 3 to 6 minutes. Stir in the reserved diced tomatillos and the remaining ½ teaspoon salt, cover, reduce the heat to medium-low and cook, stirring occasionally, until the tomatillos start to break down and become jammy and the turmeric is tender with a slight bite, 4 to 7 minutes more. 
4. Turn off the heat and stir in the fresh lemon juice. Taste and season with more salt or lemon juice, if necessary. Let cool to room temperature and serve or transfer to an airtight container and store in the refrigerator for up to 1 week.

Recipe Note: If you can’t find mustard seed oil, you can use any neutral oil, like canola or untoasted sesame oil, as a base. To get that mustardy punch, add 1 teaspoon Coleman’s ground mustard when adding the chili powder.