In a large pot over medium heat, melt butter or warm olive oil.
Add diced onion and cook until soft and translucent, about 4–5 minutes.
Stir in garlic and cook for 30 seconds until fragrant.
Add crushed tomatoes and tomato juice. Stir well and bring to a gentle simmer.
Simmer uncovered for 15–20 minutes, stirring occasionally.
Use an immersion blender (or carefully transfer to a blender) and blend until smooth.
Stir in heavy cream and season with salt and pepper to taste.
Add a pinch of sugar only if needed. Heat gently — do not boil.