This recipe from chef Cheetie Kumar of Garland in Raleigh, N.C., who replaced mango in mango pickle with green tomatoes and watermelon rinds. Notes: for best results, prepare two days before eating. Mustard oil is sold at Indian markets; it cannot be substituted and should not be omitted.
Prepare the tomatoes & watermelon rinds: Cut the tomatoes and rinds into batons; keep separate. Toss the tomatoes with ¼ cup salt, then place in a colander for 3 to 4 hours or refrigerate overnight. Toss the rinds with the remaining 2 tablespoons salt and cover with cold water; refrigerate overnight.
The next day, drain any juices from the tomatoes. Set aside.
Then drain and rinse the rinds and boil the rinds in fresh water for 5 minutes. Drain and rinse the rinds once more. Combine the vinegars and sugar; bring to a boil, then reduce the heat to medium-low. Add the rinds to the brine and cook until clear. Remove from the heat and drain, discarding brine.
Make the achaar: In a wok or deep, nonreactive skillet, add both oils and heat until very hot but not smoking. Working quickly, add the fenugreek, immediately followed by the nigella and fennel. As soon as they are sizzling, add the turmeric, chili powder, jaggery, and salt. Stir well until dissolved and fragrant. Add the rinds and cook for 2 to 3 minutes. Add the tomatoes and continue cooking for 1 to 2 minutes more. Add the vinegar and bring to a boil over high heat. Transfer to a clean, dry heatproof bowl and let cool to room temperature. (The pickle will taste very salty at this point; it will balance out in 24 to 36 hours.) For best results, place the pickle in a resealable plastic bag with all the air pushed out, lay flat in a container, and refrigerate for up to 2 weeks.
Yield: makes 1 quart