Press Sauté. Melt the butter, then add the onion and cook 4–5 minutes until soft and translucent.
Add the garlic and tomato paste. Cook 1 minute, stirring, until the paste darkens.
Pour in the broth and scrape the bottom of the insert completely clean. Press Cancel.
Add the tomatoes with their juices, the sugar, and the salt. Do not stir — leave the tomatoes sitting on top of the broth.
Lock the lid, set the valve to Sealing, and cook on High Pressure for 8 minutes.
Let the pressure release naturally, about 15 minutes.
Stir in the baking soda. Blend with an immersion blender until completely smooth, 60–90 seconds.
With the pot off, slowly stir in the milk. Taste and adjust salt and sugar. Serve hot.