In a medium-sized heavy-bottomed saucepan, heat the lard to rippling over medium-high heat. Add the onion and garlic; cook, stirring occasionally, until the onion is translucent, about 3 minutes.
Break up the tomatoes coarsely with your fingers. Add the tomatoes and their juices to the sauteed onion. Stir in the chiles and oregano. Reduce the heat to medium and cook, covered, stirring occasionally, for 15 minutes or until slightly concentrated.
Remove from the heat and let cool to lukewarm, about 15 minutes. Puree the mixture in a blender or food processor. Taste for salt (the tomatoes will have some) and add a little if needed. Serve hot at once or store tightly covered in refrigerator for up to 4 or 5 days and reheat before serving.