Heat oil in a large saucepan over medium heat; cook and stir onion, jalapeno pepper, and garlic until onion is softened, 5 to 10 minutes. Add tomatoes, tomatillos, avocado, and water to onion mixture; bring to a boil. Reduce heat and simmer until tomatoes are softened and liquid has evaporated, about 15 minutes. Remove saucepan from heat and cool slightly.
Blend tomato mixture, sour cream, cilantro, and salt together in a food processor or blender, working in batches if necessary, until sauce is smooth.
