Fabulous French tuna sandwich

Ingredients

• 4 onion or kaiser buns , halved
• 2 tsp olive oil
• 1 large onion , finely chopped
• 1/4 tsp granulated sugar
• 1/4 tsp salt
• 1/4 tsp ground black pepper
• 1 to 2 tbsp white vinegar
• 2 tsp dried tarragon leaves
• 1/3 cup mayonnaise , preferably light
• 2 tbsp pitted and finely chopped black olives , optional
• 2 oil-packed sun-dried tomatoes , halved, and finely chopped
• 170-g can chunk light tuna
• 2 cups chopped spinach , or arugula