In a medium-size (4- or 5-quart) pot (preferably a Dutch oven or Mexican cazuela), heat the oil or lard over medium heat. Add the onion and cook, stirring regularly, until golden, about 7 minutes. Raise the heat to medium-high, and, when noticeably hotter, stir in the tomato puree. Cook, stirring, until darker in color and thickened to the consistency of tomato paste, about 10 to 15 minutes. Stir in the broth, partially cover and simmer 15 minutes. Taste and season with salt, usually about 1/2 teaspoon. The sauce should be a slightly soupy consistency—not as thick as spaghetti sauce. If it is too thick, stir in a little additional broth. Keep warm over low heat.