Heat oil in a pan; add the onions and cook on medium flame, stirring occasionally until the onions are caramelized around the edges. This will take about 12~15 minutes. Season with salt and pepper; set aside to cool slightly.
Butter the bread slices generously and build the sandwiches butter side out.
On one slice, sprinkle the cheese, top with caramelized onions, tomato slices, salt and pepper, more cheese.
On the other slice, spread the chutney and place it on top of the other slice. Press lightly and put the sandwich on a grill or a panini press. If using a grill, weigh the sandwich down with a heavy plate or pan and cook for 1~2 minutes. Grill until golden brown, flip and repeat on the other side. Serve right away.