Start by rinsing the buckwheat groats under cold water in a fine mesh strainer. Drain and set aside.
Set your Instant Pot to the sauté function. Once hot, add the olive oil and let it heat for a minute.
Add the chopped onion and sauté for about 3-4 minutes until it becomes translucent.
Stir in the minced garlic and sauté for an additional 30 seconds until fragrant.
Add the diced carrot and zucchini, and sauté for another 2-3 minutes.
Pour in the vegetable broth and add the rinsed buckwheat, diced tomatoes (with their juice), kale, oregano, basil, and red pepper flakes (if using).
Season with salt and pepper to taste, then stir until well combined.
Close the Instant Pot lid and make sure the valve is set to sealing. Select the manual or pressure cook setting and set the timer for 15 minutes.
Once the cooking time is up, let the pressure release naturally for about 10 minutes before carefully switching the valve to venting to release any remaining pressure.
Open the lid, stir the soup, and taste for seasoning, adjusting salt and pepper as needed.