Rinse chickpeas under cold running water. Place in a bowl and cover with at least 4 cups of cold water. Soak for 12 to 24 hours.

Drain chickpeas and place in a large pot. Pour in enough water to cover chickpeas by 2 inches. Add halved onion and bring to a rolling boil over medium-high heat. Reduce heat to a high simmer. Cook for a few minutes until a foam forms at the top. Skim off the foam and cook, covered, until chickpeas are soft, 40 to 60 minutes.

Remove from heat, season with salt, and set aside. Do not drain so chickpeas don't dry out.

Heat oil in a large pot over medium heat. Add chopped onion and ginger. Season with salt and pepper and cook, stirring occasionally, until onion is soft and translucent, about 5 minutes. Add garlic and cook for 1 more minute. Stir in turmeric, cumin, and red pepper flakes.

Drain chickpeas, reserving 2 cups of the cooking liquid. Add drained chickpeas to the sauteed onion mixture and stir until well coated with the spices. Cook until chickpeas are a little bit crisp, stirring occasionally, 8 to 10 minutes.

Add 2 cups of chickpea cooking liquid and coconut milk to the pot. Bring to a simmer, scraping up any browned bits from the bottom of the pot. Cook, uncovered, stirring occasionally, until curry has thickened to your desired consistency and flavors have started to come together, 40 to 60 minutes. Taste and season with salt and pepper.

Divide curry amongst bowls and top with yogurt and cilantro.