Add onion, garlic, salt, and pepper to a food processor and pulse to chop finely.
Heat olive oil in a saucepan over medium heat. Transfer onion mixture to the saucepan and cook for 5 minutes to soften. Add diced tomatoes (with juice) and vegetable broth to saucepan. Increase heat and bring to a gentle boil for 10 minutes.
Remove from heat, then add heavy cream and blend using an immersion blender. Top with sliced basil.