Sort and wash peas; place in a medium bowl. Cover with water to 2 inches above peas; cover and let stand 8 hours. Drain peas.
Combine peas, broth, onion, and 2 garlic cloves in a large saucepan; bring to a boil. Partially cover, reduce heat, and simmer 1 hour or until tender. Drain.
Place poblano pepper on a foil-lined baking sheet; broil 15 minutes or until blackened and charred, turning occasionally.
Reduce oven temperature to 375°.
Place pepper in a zip-top heavy-duty plastic bag; seal. Let stand 15 minutes. Peel and discard skins. Cut a lengthwise slit in pepper; discard seeds and stem. Coarsely chop pepper. Combine remaining 2 garlic cloves, pea mixture, pepper, and salt in a food processor; process until smooth. Add sour cream; process until blended. Stir in green onions. Spoon mixture into a 9-inch pie plate or shallow 1-quart baking dish coated with cooking spray. Bake at 375° for 15 minutes or until thoroughly heated. Sprinkle with cilantro.