Preheat the oven to 350°F (medium temperature).
Peel and quarter the beets. Take a large piece of aluminum foil and place it on a baking sheet. Arrange the beets in the center and drizzle with olive oil and a pinch of salt. (You may want to wear gloves for this as beets do stain your hands).
Fold and seal the aluminum foil to create a pouch. Place it in the oven and roast for 40 minutes until the beets become tender – to check for doneness, pierce the beets with a knife.
Wash, Peel and finely chop the onion and the carrot. Wash, discard the leaves, and finely chop the celery stalk (if using).
Once the beets are tender, remove them from the oven and set aside.
Turn your Instant Pot to "saute" and add the butter, onion, carrot, and celery, season with a pinch of salt, and sauté, stirring often, for about 10 minutes or until the vegetables become very soft.
Add the roasted beets, sugar, and beef broth, close your Instant Pot lid, ensuring the valve is set to "sealing". Use the "pressure cook" or "soup" function and set the timer for 10 minutes. When the pressure cooking cycle is complete, let the pressure release naturally for 15 minutes. Then, flip the vent to "venting" and release the remaining pressure. (if you try to open it too soon, it can splatter everywhere)
Transfer the vegetables and broth to a blender or use an immersion blender. Add the lemon juice, and blend until the soup is smooth.