Step 1

Preheat oven to 375 degrees F (190 degrees C). Place the potatoes and beets into a baking dish, and drizzle with 1 tablespoon olive oil; season with salt and black pepper.

Step 2

Roast in the preheated oven until tender, about 1 hour; allow to cool.

Step 3

Heat 2 tablespoons of olive oil in a large soup pot over medium heat, and stir in the celery, onion, and garlic. Cook, stirring often, until the onion is translucent, about 5 minutes. Mix in the cooked vegetables, chicken broth, thyme, and bay leaf, bring to a boil, and reduce heat. Simmer until the celery is very tender, about 45 minutes.

Step 4

Remove from heat, and discard the bay leaf. Puree the mixture in the pot with an immersion blender until very well blended; stir in the milk and lemon juice to serve.