Step by step

1. For the Spice Mixture: Combine cumin, coriander, cardamom (if using), and dried habanero (if using) in a small saucepan and set over medium heat. Toast spices until fragrant, about 2 minutes. When cool enough to handle, remove cardamom pods, then place remaining spices in spice grinder or mortar. Using fingers, crack open cardamom pods and remove seeds contained within; discard hull. Add cardamom seeds to other spices. Using spice grinder or mortar and pestle, grind spices to fine powder. Set aside.
2. For the Okra: Heat canola oil in Dutch oven or 3-quart saucepan over medium heat until shimmering. Add onions and salt and cook, stirring occasionally with a wooden spoon, until onions are golden and just starting to brown, about 15 minutes.
3. Stir in garlic and ginger and cook until fragrant, about 30 seconds. Stir in tomato paste, stir and cook until paste has colored the contents of the pot a reddish-brown, about 1 minute. Mound onions, ginger, and garlic in center of pot and add spice mixture directly on top (this is to guard against scorching the spices). Stir together, then cook until spices are bloomed and mixture is very fragrant, 30 seconds to 1 minute. Add water and stir, scraping bottom of pot with wooden spoon to dislodge any stuck-on bits. Add tomato sauce and okra and stir to break up frozen okra. (Okra pieces should be just submerged in the liquid; if not, add more water to cover them). Bring contents of pot to a boil, reduce heat to low, and simmer, stirring occasionally, until okra is tender, most of excess liquid has cooked off, and okra is thickly coated in sauce, 10-15 minutes. Top with cilantro and serve immediately as part of a larger meal with rice or paratha, passing lime wedges alongside.