In a medium skillet, heat the oil over medium heat and cook the onion and garlic, stirring, until they begin to brown, 4 to 5 minutes.
Reserve in the pan off heat.
Prepare the tomatoes.
Put the tomatoes with skins and juices in a blender.
Add the serrano chiles and blend until smooth.
Transfer the sauce to the skillet with the onion and cook over medium-low heat, partially covered, stirring frequently, to blend flavors, about 10 minutes.
Add salt.
Notes / Tips / Wine Advice:
The sauce is ready to use, or store the sauce in a covered container, refrigerated, up to 3 days, or freeze up to 3 months.