Heat oven to 425°F. Line a baking sheet large enough to hold the beets and onions in a single layer with parchment paper. Place the 1 pound peeled and chopped beets on the baking sheet and drizzle with 1/2 tablespoon of the olive oil. Toss the beets in the oil and put in the oven to roast for 30 minutes. Remove from the oven, add the onions toss everything with another 1/2 tablespoon of oil and sprinkle with the 1/2 teaspoon of kosher salt and 1/4 teaspoon ground black pepper. Toss until vegetables are evenly coated. Spread into a single layer. Roast vegetables another 15 minutes or until a fork easily goes through a beet. Lower the oven temperature to 350°F to make the croutons.
Place caraway seeds in a mortar or molcajete and crush into a powder or use ground caraway if you can find it.
Combine bread cubes the remaining 2 tablespoons of olive oil, the ground caraway seeds, and 1/4 teaspoon kosher salt on a baking sheet. Toss until well combined. Then spread into an even layer.
Bake until crisp and golden brown, about 15 minutes, stirring halfway through. Let cool.
Meanwhile, blend the soup. Combine cooled beets and onions with 1 1/2 teaspoons kosher salt, 3 cups cold water, and 3 tablespoons balsamic vinegar in a blender. Start blender on low then gradually increase the speed to high and blend for 1 minute. You may need to do this in batches if you have a smaller blender.
While you can enjoy this right away it is best if it has a few hours to chill in the refrigerator and let the ingredients meld and blend.
To serve, ladle borscht into small bowls and top with a few croutons, a swirl of sour cream, and a sprinkling of chopped dill.