Saute the onion in large skillet with 1 Tbsp butter for 1 minute. Add garlic and carrots. Cook, covered, for 5 minutes. Add 1 cup chicken broth and continue to cook covered for 10 minutes. Transfer mixture to a blender and blend until smooth. Pour into a big saucepan.

Meanwhile, boil a small pan of water. Trim, peel, and dice the kohlrabi. Peel and dice the sweet potato. Wash the kohlrabi leaves then boil for 1 minute. Drain, cool, and chop. Set aside.

Once the broth is pureed, add the remaining 2 1/2 cups chicken broth, water, kohlrabi, and sweet potato. Cook until veggies are tender, about 15 minutes.

In a small saucepan, melt 2 Tbsp butter. Stir in flour and cook for 2 minutes. Whisk in some hot soup. Whisk mixture together then pour back into the rest of the soup. Cook until slightly thickened, 10 minutes. Add kohlrabi leaves. Cook for another 5 minutes. Add lemon juice, salt, and pepper to taste.