1. Prepare grill.
2. Prick sausages in several places with a fork. Grill, turning occasionally, 8 minutes or until hot throughout.
3. Combine tomatoes, onion, 1/2 tablespoon oil, and 1/4 teaspoon each salt and pepper. Grill 5–6 minutes per side for onion and 2–3 minutes per side for tomato or until charred and tender. Transfer to a large bowl; toss with spinach.
4. In a small bowl, whisk together mustard, vinegar, remaining 1 tablespoon oil, and remaining 1/4 teaspoon pepper. Slice sausages, and serve with vegetables and dressing.