Add butter to dutch oven or heavy stock pot over medium heat and melt. Add onion and garlic. Cook until onion is soft and fragrant, about 10 minutes. Stir to make sure onions/garlic don't burn.
Increase heat to medium-high and add the tomato paste. Continue to cook, stirring constantly until the mixture has started to caramelize in spots - about 5-6 minutes. It will smell amazing but be careful not to burn it.
Add tomatoes with juices, 1 teaspoon of sugar, and 8 cups of water to the pot. Stir with a wooden spoon to make sure everything is combined. Increase heat to high and bring to a simmer. Before it gets to a boil, reduce the heat to medium.
Simmer uncovered for at least 60 minutes to let flavors combine and to reduce some of the liquid.
Remove from heat and carefully use an immersion blender to break up the whole tomatoes and puree until smooth.
Return to heat and stir in 1/3 cup of Kefir. Simmer for another 10-25 minutes, adding more Kefir as desired. Season with salt, pepper, and another 1 teaspoon of sugar.
Serve with homemade crispy croutons or grilled cheese and enjoy!