Preheat the oven to 425 degrees F. Spray a large roasting pan with cooking spray.
Toss together the potatoes, 2 teaspoons of the oil, the rosemary, 1/2 teaspoon of the salt and 1/2 teaspoon of the pepper in a large bowl.
Place the sausage in the center of the roasting pan and scatter the potato mixture around the sausage. Roast, stirring the potatoes and turning the sausage occasionally, until the potatoes are tender and the sausage is cooked through, about 40 minutes. Transfer the sausage to a cutting board. Let cool 5 minutes before cutting into 1-inch slices.
Whisk together the vinegar, mustard, remaining 2 teaspoons oil and remaining salt and pepper in a large bowl. Add the potatoes, sausage, spinach, bell pepper, onions and olives and mix well. Serve warm or at room temperature.