Melt the butter in a Dutch oven over medium heat until the foaming subsides. Add the onion, fennel bulb pieces, and a pinch of salt and pepper. Cook, stirring occasionally, until the vegetables are softened and just starting to brown, about 8 to 10 minutes.

Stir in the garlic and thyme and cook until fragrant.

Stir in the flour and cook for 1 minute to coat the vegetables and remove the raw flour taste.

Whisk in the wine and simmer, scraping up any browned bits from the bottom of the pot, until the liquid has nearly evaporated.

Stir in the broth, water, celery root, potatoes, shredded apple, bay leaf, and orange zest. Increase the heat to high and bring to a boil, then reduce the heat to low and simmer until the celery root and potatoes are tender when pierced with a fork, about 20 to 25 minutes.

Discard the bay leaf and orange zest strip. Transfer 2 cups of the soup to a blender, add the heavy cream, and process until completely smooth. Return the puree to the pot and stir to combine—this creates a silky body while leaving plenty of chunky texture.

Stir in the reserved fennel fronds and season with salt and pepper to taste. Serve hot.