Heat grill to high. Brush tomatoes and serrano with oil and season with salt and pepper. Grill tomatoes and serrano on all sides until charred. Remove from the grill and coarsely chop.
Heat oil in a medium saucepan on the grates of the grill or on the side burners. Add the onion and cook until soft. Add the tomatoes, serrano, balsamic vinegar, and saffron and cook until thickened. Season with salt and pepper. Transfer to a bowl and let cool to room temperature before serving.
