Heat oil in a pan, Add in onions, capsicum and cook them till onions are caramelized. It will take around 10 mins.
Now add in mushroom and cook them down for 5 mins or so.
Now add in oregano, salt, red chilli flakes and mix well. Add in ketchup and mix well.
Take one slice of bread, place some filling in, place cheese on top. Cover with a slice of bread. Now spoon some filling in and top with cheese and place another slice of bread.
Oil a grill maker or a sandwich maker or a tawa. Place the sandwich in and grill for 3 to 4 mins till golden.