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1. Preheat the oven to 375 degrees. Coat a large baking sheet with cooking spray; set aside. Coat a baking dish with cooking spray.
2. Place the sweet potatoes cut-side down in the baking dish. Bake for 30 minutes, or until tender. Peel the sweet potatoes and place them in a medium bowl. Mash with the olive oil, black pepper and ¼ teaspoon of the salt. Stir in the pecans.
3. Place the chicken breasts in the baking dish and rub with the garlic, rosemary, and ¼ teaspoon of the salt. Bake for 15 to 20 minutes, or until a thermometer inserted in the thickest portion registers 160 degrees and the juices run clear.
4. Meanwhile, place the collard greens, tomatoes, onion, and oregano on the baking sheet and coat them with cooking spray. Toss and spread out on the sheet. Bake with the chicken for 15 minutes. Sprinkle with the vinegar and the remaining ¼ teaspoon salt. Return to the oven and bake for 5 minutes.
5. Divide the chicken, sweet potatoes, and collard greens among four plates.