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1
Preheat oven to 325F.
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2
If using peanuts, blend in a food processor until they form a butter, 2 to 3 minutes.
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3
Put fresh or jarred peanut butter in a bowl and gradually whisk in 1 1/2 cups water.
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4
Pat chicken dry and season with salt.
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5
Heat oil in an ovenproof 4- to 5-quart heavy pot (with a tight-fitting lid, for use later) over moderately high heat until hot but not smoking, then brown chicken, uncovered, in 3 or 4 batches, without crowding, about 6 minutes.
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6
Transfer chicken to a bowl as browned.
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7
Pour off all but about 2 tablespoons fat from pot, then add onion and bell pepper and saute, stirring occasionally, until onion begins to brown, about 4 minutes.
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8
Add garlic and saute, stirring, 1 minute.
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9
Stir in peanut butter mixture, remaining 1 1/2 cups water, tomatoes with juice, cayenne (to taste), salt, and chicken with any juices accumulated in bowl and bring to a simmer.
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10
Cover pot with lid, then braise chicken in middle of oven until tender, 45 minutes to 1 hour.
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11
Transfer chicken with tongs to a large (4-quart) serving dish and keep warm, covered.
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12
Stir potatoes and turnips into sauce and simmer on top of stove, uncovered, until vegetables are tender, 15 to 20 minutes.
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13
Transfer cooked vegetables with a slotted spoon to serving dish.
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14
Simmer sauce, uncovered, stirring, until reduced to about 4 cups, about 5 minutes.
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15
Remove from heat and stir in spinach, then let stand, partially covered, until spinach is wilted, 2 to 3 minutes.
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16
Season with salt if necessary, then spoon over chicken.
