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Make the chickpeas: If using dried chickpeas, place in a large bowl and pick through and discard any stones or shriveled chickpeas. Cover with 3 quarts cold water. Stir in 1 tablespoon of kosher salt and let sit at least 10 hours or overnight at room temperature.
Heat the oil in a large frying pan over medium-high heat until shimmering. Add the onion, garlic, and ginger, and cook, stirring occasionally, until the onions are tender, about 5 minutes.
Add the mustard seeds, cumin, turmeric, coriander, and cardamom and cook, stirring, until fragrant, about 30 seconds. Add the water and scrape up any browned bits from the bottom of the pan. Transfer the mixture to a blender and blend until smooth (reserve the blender, no need to wash). Transfer to a 4-quart or larger slow cooker.
Add the coconut milk and 2 teaspoons salt. Add the canned chickpeas, or drain the soaked chickpeas and add to the slow cooker. Stir to combine.
Cover and cook on the LOW setting until the chickpeas are tender, 6 to 8 hours for canned or 8 to 10 hours for dried. Meanwhile, place the tomatoes in the blender and blend until smooth. Refrigerate until the chickpeas are ready.
Add the blended tomatoes and stir to combine. Cover and cook until the flavors meld, at least 30 minutes but up to 1 hour. Just before serving, make the tarka.
For the tarka: Heat the oil in a small frying pan over medium-high heat until just starting to smoke. Add the cumin seeds, red pepper flakes, and mustard seeds and fry, swirling the pan, until sizzling and very fragrant, about 20 seconds. Immediately scrape the tarka into the chickpeas.
Add the spinach and gently stir to combine and wilt. Taste and season with salt as needed. Serve warm over rice, with lime wedges on the side.