The trick to making a rich and thick tomato sauce is all in the process. You will need a deep and heavy bottomed pot to cook the sauce and you will need to simmer the sauce for much longer than you expect. About two hours will do it, but a while longer on the stove will make the sauce as thick and flavorful as possible. Just remember to stir the pot occasionally while cooking so the sauce on the bottom of the pot does not stick or burn.
In a large pot, heat olive oil over medium heat. Stir in the red onion and chopped garlic and cook for 10 to 15 minutes or until they begin to soften and brown. Stir frequently. Stir in the tomatoes, mixing together the ingredients. Add in the basil, oregano, cilantro and marjoram, stirring until combined. Cook for about 3 minutes or until ingredients begin to bubble. After reducing the heat to medium-low, cook the sauce for 2 to 3 hours. Make sure to stir occasionally while the sauce simmers and thickens. When sauce reaches desired thickness, season with salt and pepper.