Heat the oil in a 1- or 2-quart saucepan over medium heat until shimmering. Add the onion and cook, stirring frequently, until slightly translucent, about 4 to 5 minutes. Add the tomatillos, tomatoes and jalapeño peppers.
Continue to cook, stirring occasionally, for 10 to 12 minutes, until the vegetables are soft. Cool. Transfer to a blender or food processor. Add the garlic, cilantro, and salt. Process until smooth.
Serve with rice, grilled meat and vegetables, tacos, burritos and tortillas.
The salsa will keep for up to a week in the refrigerator. It may also be processed in jars.