Adapted from the Silver Palate Cookbook. It isn't really spicy -- I think they call it spicy because it has a lot of black pepper in it. I think the best and fastest way to do sauce like this is to put the cooked sauce through a Foley Food Mill. If you prefer to get rid of the skin and seeds ahead of cooking, there are a few methods, which can be found in Vegetables 101.
1/4 cup EVOO
2 cups yellow onions, diced
6-7 pounds plum or other tomatoes (equals about 4 quarts of farm plum tomatoes), cut in large chunks
2 large sprigs of basil, roughly chopped
2 Tablespoons dried oregano
1 can tomato paste
2 Tablespoons black pepper
1 Tablespoon salt
1 large garlic clove, minced
1/4 cup (or more to taste) chopped parsley
Heat olive oil in a large, heavy pot over medium-low heat. Add onions and cook very slowly, covered, until translucent -- do not let brown. This will take about 20 minutes. Raise heat to medium and add tomatoes. When juices start to release, add tomato paste, basil, salt and pepper and oregano. Some tomatoes release a lot of juice and others very little, so you may or may not need to add water. Cook over low heat (keep at a simmer), uncovered for about 2 hours, stirring frequently. Add chopped parsley and minced garlic 5 minutes before serving.
This sauce cans and freezes very well. Add parsley and garlic when reheating.