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Combine chicken, potatoes, onion, and garlic in a 6-quart slow cooker.
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Whisk together lemon juice, olive oil, Dijon mustard, oregano, rosemary, salt, pepper, and crushed red pepper in a small bowl. Drizzle over chicken and vegetables and toss to coat.
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Cover and cook on Low for 6 to 7 hours or on High for 2 ½ to 3 ½ hours, or until an instant-read thermometer inserted into the thickest part of a chicken thigh reads at least 165 degrees F (74 degrees C).
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Optional step (to brown chicken and potatoes): Preheat the broiler. Line a rimmed baking sheet with aluminum foil. Transfer chicken thighs to the prepared baking sheet and place 4 to 5 inches from the heat. Broil until browned, 2 to 3 minutes. Transfer to a serving dish.
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Transfer vegetables with a slotted spoon to the baking sheet. Broil until browned, 2 to 3 minutes. Add to the serving dish with the chicken.
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Drizzle with cooking liquid. Garnish with lemon zest.