Wash tomatoes, cut each in half. Have a shallow saucer of olive oil near your cutting board. Take each half, dip the cut surface in the olive oil, and place on a cookie sheet-- best to use the kind with a little "lip", because you want to keep the liquid. Roast in the oven, 375 degrees, for 45 minutes.
In the meantime, chop the onion, garlic, and the herbs if using fresh.
When the tomatoes still have about 5 minutes left in the oven, start cooking the onion in olive oil over medium heat. Use a large, deep saucepan, at least 3 quarts. When onions are turning a little translucent, add the garlic.
Place the roasted tomatoes in the pan, and be sure to include the liquid from the cookie sheet as well. Stir until the tomatoes break up a little, add the herbs and the sherry.
Allow to cook over medium heat, stirring often, for about 15 minutes. Covering the saucepan cuts down on the mess, but don't forget to stir!
At this point add the balsamic vinegar, and, if desired, you can make the sauce less "chunky"... I use a potato masher break up the tomatoes for a chunky sauce, or an immersion blender to make a smooth sauce. Be sure to turn the heat to low if you're going to use the immersion blender.
Stir, add salt and pepper to taste, and cook for another 10-15 minutes or so.
Prepare your favorite pasta, mix the sauce in, and serve!