In a 6- to 8-quart pan, combine onion, tomatillos, chilies, garlic, and 2 1/2 cups water.
Bring to a boil over high heat; reduce heat, cover, and simmer until vegetables are tender when pierced, about 20 minutes. Drain.
In a food processor or blender, smoothly purée vegetable mixture with romaine, cilantro, and radish leaves.
Pour about half the purée into a bowl; set aside.
Add 1 1/2 cups pumpkin seeds and 1/2 cup broth to purée in food processor; whirl until mixture is smooth. Pour into cooking pan.
Pour purée from bowl back into food processor. Add peanuts, sesame seed, and 1/2 cup broth and whirl until smooth. Add to pan.
Add 3 cups broth to pan. Bring to a simmer over medium heat. Cover and cook over low heat, stirring occasionally, until sauce thickens, about 30 minutes. Sauce will look curdled.
In a food processor or blender, whirl about half the sauce and 1 cup broth until smooth. Pour into a fine strainer over a bowl, pressing sauce through with the back of a spoon; discard residue.
Purée remaining sauce and 1 cup broth, and rub through strainer. (If making ahead, cover when cool and chill airtight up to 2 days.)
Return sauce to pan and stir over medium heat until hot.
Ladle about 3/4 cup sauce onto each dinner plate; lay chicken in sauce and garnish with cilantro sprigs and remaining pumpkin seeds. Accompany with remaining sauce and salt and pepper to add to taste.