Carefully pick through peas to remove any debris or small stones and rinse well. Transfer to a large bowl, cover by 3 inches with water, cover and set aside at room temperature for 8 hours or overnight; drain and rinse well.
Heat oil over medium heat in a large pot. Add onion, garlic, ham and celery and cook until onion is translucent, 5 to 8 minutes. Add peas and broth, bring to a boil and skim off and discard any foam on the surface. Reduce heat and simmer, partially covered, until peas are tender, about 45 minutes. Add collard greens and carrots and simmer until tender, 15 to 20 minutes more. Season with salt, pepper and cayenne and serve.
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