Heat olive oil in a large pot over medium heat. Add diced onion and sauté until translucent, about 5 minutes. Stir in minced garlic and cook for an additional minute until fragrant.

Incorporate diced beetroots, carrots, celery, and potato into the pot. Stir the vegetables to coat them with the oil and aromatics.

Add vegetable broth to the pot, ensuring the vegetables are fully submerged. Sprinkle in dried thyme, salt, and freshly ground black pepper.

Bring the mixture to a boil. Reduce the heat to low, cover the pot, and let it simmer for about 20-25 minutes, or until the vegetables are tender.

Using an immersion blender, puree the soup until smooth. Alternatively, carefully transfer the soup in batches to a blender, blend until smooth, and return it to the pot.

Stir in the lemon juice to brighten the flavors. Taste and adjust seasoning with salt and pepper if necessary. Ladle the soup into bowls and garnish with chopped fresh parsley.