Heat grill to medium-high. With a fork, prick the sausages in several places. Grill, turning occasionally, until cooked through, 10 to 12 minutes.
Meanwhile, brush the tomatoes and onion with 1 tablespoon of the oil and season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Grill until tender, 5 to 6 minutes per side for the onion and 2 to 3 minutes per side for the tomatoes. Transfer the vegetables to a large bowl as they are done, add the spinach, and toss.
In a small bowl, whisk together the vinegar, mustard, the remaining 3 tablespoons of the oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Slice the sausages and serve with the vegetables and dressing.
Tip: Tossing the spinach with warm vegetables wilts the leaves slightly and makes them more tender. Delicious on its own, this salad is also great mixed into pasta.