Rinse chilies, pat completely dry with a clean towel, and air-dry on a rack at least twenty minutes. Wearing gloves if you are sensitive, trim stems and make a lengthwise slit in each chili without cutting all the way through, so the seeds stay mostly inside. Place in a wide dry bowl, toss with salt and turmeric, and set aside while you heat the oil.

In a small pan, bring mustard oil to a light haze (if using mustard oil) or until very hot for neutral oil, then lower heat. Add hing; it should foam gently. Off heat, add cracked mustard and fenugreek, swirling the pan a few seconds until fragrant, not black. Let the oil cool until warm, not scorching, about five to eight minutes.

Pour the warm oil over the chilies. Add lemon juice and optional sugar. Toss well, taste carefully, and add a pinch more salt or lemon if needed. Pack into a very clean dry glass jar, pressing gently to submerge as much as possible. Cool completely before sealing.

Refrigerate at least overnight before first taste; flavor rounds after two to three days. Serve a few chilies with thepla, dal, or khichdi, keeping the rest refrigerated.