Add the heirloom tomatoes, 1 teaspoon of salt, and broth. Bring to a boil; then lower the heat and simmer (uncovered) for 15 minutes.
Place the heirloom tomato mixture into a blender and blend until smooth.
For the roux: In the same large stockpot (that is now empty), melt the butter over medium-high heat. Add all the flour and stir vigourously for 2-3 minutes. As the mixture comes together and begins to bubble, lower the heat and whisk slowly for another 2 minutes.
Add some of the heirloom tomato mixture (about a cup) back to the pot (this gradual process helps to avoid lumps). Pour in the rest of the heirloom mixture, stir and simmer on low heat for another 5 minutes.
Add the sweetener and stir to combine. Do a final test for salt to taste.
Serve with optional toppings and enjoy!
Recipe by Six Snippets at https://www.sixsnippets.com/heirloom-tomato-soup-goodbye-summer-already/