Pick over and rinse the dried red kidney beans. Place in a bowl and cover with water (for soaking) by at least 2 inches; soak 8–12 hours, then drain.
Transfer the soaked beans to a large pot, add 8 cups water and the bay leaf, and bring to a boil. Reduce to a gentle simmer and cook until the beans are very tender, 60–90 minutes. In the last 15 minutes of cooking, season with 1.5 tsp kosher salt. Reserve 1 cup cooking liquid, drain the beans, and discard the bay leaf.
While the beans cook, warm the vegetable oil in a skillet over medium heat. Add the finely chopped onion and cook, stirring, until soft and golden, 8–10 minutes; remove from heat.
In a mortar or food processor, combine the walnuts, garlic, ground coriander, blue fenugreek (utskho suneli), dried marigold (imeretian saffron), red pepper flakes, and the remaining 0.5 tsp kosher salt. Process to a coarse paste, then blend in the red wine vinegar and 0.5 cup hot bean cooking liquid to loosen.
Return the drained beans to the pot. Stir in the sautéed onions and the walnut-spice paste.
Mash about half the beans with a potato masher until thick and creamy, adding more cooking liquid as needed (up to 0.5 cup) to achieve a spoonable consistency.
Set the pot over low heat and simmer, stirring often, 3–5 minutes to meld the flavors.
Stir in the finely chopped cilantro. Taste and adjust seasoning.
Let rest off heat for 10 minutes. Serve warm or at room temperature, topped with the thinly sliced onion (for serving).