Heat 2tsp oil in a nonstick pan, add the onions and cook on medium flame until browned and caramelized. Make sure not to burn them.
Spread mayonnaise on both sides of the bread and top it with caramelized onions, cheese and tomato.
On the other bread, spread mayo on one side and green chutney on the other side. Place the green chutney side on the tomato and press lightly.
Cook on a hot nonstick pan pressing lightly. Cook till both sides are golden and toasty and the cheese is melted. Serve hot with the tomato soup.