Brush the tomatoes and serrano with 1 tablespoon of vegetable oil, and sprinkle with salt and pepper. Grill the tomatoes and chile on all sides until you get a char. This won't take long. Remove from the grill and let it cool until you can handle them. Give the tomatoes and chile a rough chop. (I seed my pepper to cut the heat level. Remember to wear gloves if you do!).
Heat up the remaining 2 tablespoons of vegetable oil in a medium saucepan. Toss in the onion and cook until the onion becomes soft. Add in the chopped tomatoes, chile, balsamic and saffron. (Sub a bit of turmeric if you don't have saffron.) Cook the mixture down until it gets thick. Allow the mixture to cool to room temperature before you serve. After the mixture was cooled, I took an immersion blender and blended it just a bit, so it was a bit smoother but still had chunks, as a personal preference.