For dressing, in a screw-top jar combine the 2 tablespoons oil, the vinegar, chives or green onions, basil, sugar, mustard, and black pepper. Cover and shake well. If desired, chill for up to 24 hours.
Generously brush sweet pepper rings with olive oil. Place sweet pepper rings on the grill rack directly over medium-hot heat. Grill for 5 to 6 minutes or until crisp-tender, turning once halfway through grilling. If desired, chill for up to 24 hours.
To serve, line 4 to 6 salad plates with spinach. Arrange the sweet pepper rings and tomato wedges on spinach. Sprinkle with crumbled cheese. Shake dressing to mix; drizzle over salads. Makes 4 to 6 servings.