Melt the butter and add the olive oil to a large pot.
Sautee onions and celery for about 10 minutes, adding salt (and pepper, if desired) halfway through. Onions and celery should be soft and beginning to brown
Add the garlic, thyme, and rosemary and cook for another minute or two.
Add the celery root, turnip, and carrots to the pot. Stir to combine and cook for another 2 minutes.
Add stock, the half lemon (don’t squeeze it), and the parmesan rind if using.
Cover and simmer for 30 minutes or until root vegetables are easily pierced with a fork.
Remove lemon half and parmesan rind.
Using an immersion blender, puree the soup.
Serve topped with a drizzle of olive oil or cream and a small squeeze of additional lemon juice if desired. Also delicious with Aleppo pepper and crumbled bacon on top!