Brush one side of each bread slice lightly with oil. Brush other side of each bread slice with honey mustard or salad dressing. Lay the French bread slices down the the mustard facing up. Top with cheese. Top cheese with cucumber, spinach, and red onion. Top with wheat bread slices, mustard-side down.
Preheat an indoor electric grill or preheat a large skillet over medium heat. Place the sandwiches on the grill rack. If using a covered grill, close lid. Grill sandwiches until bread is golden and cheese is melted. (For a covered grill, allow 3 to 5 minutes. For an uncovered grill or skillet, allow 6 to 8 minutes, turning once halfway through grilling.) Cut each sandwich into 4 slices.
Meanwhile, in a medium saucepan stir together soup, tomatoes, and balsamic vinegar. Heat through. Ladle soup into serving bowls. Top each with 1 tablespoon yogurt or sour cream; swirl slightly. Sprinkle each serving with chives. Serve soup with sandwiches. Makes 4 servings.