Dice onion and sweat with a pinch of salt in large dutch oven or instant pot over medium heat/saute high for 4-5 minutes.
Add in the garlic and optional tomato paste, stir for an additional minute.
Stir in the crushed tomatoes, chicken stock, and basil (if fresh, leave whole, it will be coming out). Simmer for 10-15 minutes.
Remove the fresh basil. Add in the butter, and blend soup in countertop/immersion blender for 1 minute, or until mostly smooth.
Bring soup back to temperature and serve, garnished with more basil and a swizzle of olive oil.