Heat the oil in a saucepan over medium heat. Add the bell peppers, carrots, celery, onions, garlic and salt. Saute until veggies are soft, about 5 minutes. Add the tomato paste and cook for 1 more minute, stirring constantly.
Add the tomatoes, reduce heat to low, and simmer for 10 minutes.
Remove from heat, puree with an immersion hand blender (or in a regular blender) until almost smooth.
Return sauce to the heat and simmer until thick, 10-20 minutes. The longer you simmer this sauce the thicker and more flavorful it will become. You can simmer up to 2 hours.