How to Make It
Prepare the Masala: Heat cumin seeds in a small skillet over low, stirring occasionally, until very fragrant, about 2 minutes. Transfer to a large plate. Repeat procedure with coriander seeds, fennel seeds, and ajowan seeds, toasting each spice separately and adding to cumin seeds. Let spices cool 10 minutes. In 2 batches, process cumin mixture and, if desired, mint in a spice grinder until finely ground, about 15 seconds per batch. Transfer to an airtight container; stir in amchoor powder, black salt, and cayenne; set aside.
Prepare the Roasted Okra: Place 1 oven rack 6 inches from heat; place second rack in lower third of oven. Place a large rimmed baking sheet on each rack. Preheat broiler to high. (Do not remove baking sheets while oven preheats.)
Stir together grapeseed oil and turmeric in a large bowl. Add okra and onions, tossing to coat. Remove pan from top rack of oven. Spread half of the okra mixture (about 8 cups) in a single layer in hot pan, and return to top oven rack. Broil on high until okra is tender and charred in spots, about 10 minutes. Transfer to a large bowl. Spread remaining okra mixture in a single layer on second hot sheet pan, broiling on top rack as directed.
Gently toss cooked okra mixture with lemon juice, salt, and 4 teaspoons of the Masala. Transfer to a large serving platter.