Place cauliflower florets, 2 quartered oranges, 5 thyme sprigs, and rosemary sprigs in a 6-quart slow cooker.

Pat turkey tenderloins dry with paper towels. Place turkey in slow cooker; drizzle with olive oil. Sprinkle with salt and ½ teaspoon pepper. Add vegetable stock. Cover and cook on LOW for 2 to 3 hours or until turkey reaches 165°.

Transfer turkey to cutting board; loosely cover with foil and let stand for 10 minutes. Meanwhile, strain cooking juices; reserve 2¼ cups cooking juices and cauliflower. Discard remaining juices, oranges, and herb sprigs.

For gravy, melt butter in a small saucepan over medium heat. Whisk in flour, then slowly whisk in 2 cups reserved cooking juices. Bring to a boil; reduce heat. Simmer, uncovered, for 2 to 3 minutes or until thickened, whisking often. Remove from heat; cover to keep warm.

Place cauliflower in a food processor. Add remaining ¼ cup reserved cooking juices, sour cream, 1 teaspoon chopped thyme, and ¼ teaspoon pepper. Cover and process until creamy. Transfer to a serving bowl. Sprinkle with remaining 1 teaspoon chopped thyme and remaining ¼ teaspoon pepper.

To serve, slice turkey tenderloins. Garnish with additional thyme sprigs and orange slices, if desired. Serve with gravy and cauliflower.