Sitting under all that cheese is a simple marinara sauce. By itself it's an excellent sauce for pasta, polenta, and that Italian-American standby Chicken Parmigiana (or it's vegetarian/vegan version, Eggplant Parmigiana). It makes a great pizza sauce. I even used some as part of a seafood stew whipped up in just a few minutes. As a base, it can easily be made into another sauce with the addition of a few ingredients (anchovies and olives make a puttanesca, for example). And if you make a large batch you can put some aside to use for later, saving both time and money.
The process here is pretty simple. First we're going to perfume some oil with onion and garlic, then remove the onion from the sauce--remove the garlic as well if you wish. Next there will be some quick seasoning followed by a slow simmer of the tomatoes. Finally, the sauce gets adjusted to your taste.
¼ cup extra-virgin olive oil
1 medium onion
3 large cloves garlic
Cut the onion in half through the root and peel it. Cut each half in thirds through the root, remove the root, then separate the layers. Lightly smash the garlic and remove the paper, root end, and any green endosperm that might be present. Add the oil to a saucepan and heat over medium until the oil begins to shimmer. Add the onions and garlic and cook until the onions are translucent, about 10 minutes. Remove the onions.
1 anchovy fillet (optional)
2 tablespoons finely chopped flat-leaf parsley
2-28 ounce cans of San Marzano tomatoes (purée them)
1 tablespoon tomato paste
1 teaspoon fleur de sel or other fine sea salt
Add the anchovy if you're using it, then mash with the garlic into a paste. Add the parsley and stir for about 30 seconds, then add the tomatoes, paste, and salt. Simmer for 30-40 minutes, stirring frequently, until thickened.
2 tablespoons finely chopped fresh oregano
Add oregano and let the sauce simmer a minute or so to absorb the flavor. Some adjustments will need to be made here. If the tomatoes you used are more acidic than sweet, add 1 teaspoon of sugar. You might also need a bit more salt, particularly if you didn't use the anchovy. Some people might opt to add ½ teaspoon of red pepper flakes at this point.
If you want to set some sauce aside for another use, have a sterilized canning jar (or two) of 2-cup (0.5L) capacity handy. Ladle sauce into the jar, leaving some headroom, then seal the jar. Keep it in the refrigerator for up to 3 months, sealed. Use within a week to 10 days after opening.
Now if you want to make Chicken or Eggplant Parmigiana, all you have to do is drizzle the chicken or eggplant with some nice extra-virgin olive oil, salt it, and put it into the oven to bake until mostly done. Then spoon on some marinara sauce, grate on the cheese of your choice (parmigiano-reggiano is always my choice), then bake until done. Plate it with some starch and add more grated cheese, maybe put some salad on the side.