Preheat the oven to 450 degrees F and lightly grease a large baking sheet; set it aside.

In a large bowl, combine tomatoes, onion, and garlic. Toss with olive oil, salt, and pepper until well mixed.

Spread the tomato mixture evenly onto the prepared baking sheet and roast for 15 minutes.

Remove from the oven, stir the mixture, and roast for an additional 15 minutes, using the broiler during the last 3 minutes to lightly char some of the tomatoes.

Transfer the roasted tomato mixture to a high-powered blender, ensuring to include all contents from the baking sheet.

Add fresh basil and blend on high speed until smooth, about 50 seconds.

In a large pot over medium-high heat, melt butter.

Add flour to the melted butter and whisk until golden brown, about 30-60 seconds.

Whisk in broth and sugar until well combined.

Pour the blended mixture into the pot and whisk thoroughly. For a creamier texture, optionally add ¼ to ½ cup of heavy cream.

Cook over medium-low heat for 15 minutes, adjusting salt and pepper to taste.

Serve warm, garnished with croutons, chopped fresh basil, or other desired toppings.