Preheat oven to 400° F / 205° C.
Slice the top off the head of garlic to just barely expose the tips of the cloves. Drizzle with a little oil (or dab of oil) and wrap in parchment or aluminum foil.
To a baking sheet lined with parchment, add the quartered beets and prepared head of garlic. Roast until tender, about 30-40 mins.
In the meantime, prepare the base of the soup. Heat oil in a pot over medium heat. Add prepared leek with pinch of salt and cook until tender, about 6-8 minutes.
Add thyme and bay, and cook until fragrant, about 1-2 minutes. Remove from heat until beets are done.
Once beets and garlic are cooked and cool enough to handle, squeeze roasted garlic from the bulb into the pot with leeks and aromatics. Add the roasted beets. Return to medium-high heat.
To the pot, add 3 cups / 750 ml filtered water, bring to a boil. Season with salt and pepper, reduce heat to simmer for 5-10 mins. Once thoroughly warmed through, remove from heat, and remove the bay leaf.
Let cool slightly, and then ladle into a blender (in batches if necessary). Puree in the blender until smooth and pulse in lemon juice. Taste and add more lemon, salt, and pepper to taste.
Store in the fridge for 5-6 days, enjoy!