Peel the onions and cut into half rings. I always remove the heart of the onions, but it is certainly a matter of taste.
Melt the butter in a non-stick pan and add the half onion rings. After sweating the onions briefly, season with the tomato-mozzarella spice and the ground pepper, then add the tomato paste and soy sauce. Mix the whole thing well and, depending on your taste, continue to fry for 2 or 3 minutes over medium heat (I personally like this to be relatively dark).