Because this dish is so simple, it really relies on having the best possible ingredients. That primarily means good, ripe tomatoes. They have to be soft enough to quickly melt down to a nice, thick sauce. Underripe tomatoes won’t do that. If you’re having problems with the tomatoes cooking down, add a little water.
Paprika is an important flavoring in this dish, but notice that it is added off the heat, after the onions have been cooked. That’s because paprika easily scorches and becomes bitter.