Heat oven to 450° F. Lightly coat 2 baking pans with vegetable cooking spray. In a bowl, toss the sweet potato slices with the oil. Arrange in a single layer on one of the baking pans; sprinkle with half the rosemary. Place in the oven and cook until tender, about 30 minutes.
Meanwhile, season each side of the meat or fish with the salt and a few grinds of pepper, spread with the mustard, then sprinkle with the sugar and the remaining rosemary; arrange on the second baking pan. Place in the oven. Roast 20 minutes for the pork or chicken, or 10 minutes for the salmon, turning once, until cooked through. Check the potatoes for tenderness; let them cook longer if necessary. Arrange the potatoes on 4 plates and top with the meat or fish; cover to keep warm. Place the baking pan used for the meat or fish over medium heat on top of the stove; add the orange juice and cook until slightly thickened, about 2 minutes. Pour over the meat or fish. Season with salt and pepper to taste.