Preheat the oven to 375 degrees F (190 degrees C).

Roughly quarter the onion half, and separate the pieces. Place onion pieces on a baking tray. Open the can of diced tomatoes. Carefully drizzle some of the liquid onto the onion.

Roast onions in the preheated oven until soft, about 25 minutes. Place onions and tomatoes in a high-powered blender, such as a Vitamix. Cover tightly and blend on high for 40 seconds.

Melt butter in a saucepan over medium heat. Carefully pour in the tomato mixture, and cook, stirring occasionally until hot, about 3 minutes. Stir in 3 tablespoons cream.

Ladle soup into bowls, and drizzle remaining cream over each serving.