In a large pot, heat oil over medium heat. Add onion and cook for 2 minutes, stirring frequently. Add salt to onions, give it a stir, and add vegetables and broth.
Cover pot and lower heat to low-medium. Cook for 15 minutes and check vegetables to ensure they are soft. If needed, cook for 3-5 more minutes covered.
Turn off heat and move pot to another burner. Let cool for 5 minutes and transfer mixture to a high-speed blender.
See notes about blending (don't fill more than halfway, cover with lid and towel on top of lid) and blend until pureed. Let cool and transfer to jars.