Melt the butter in a Dutch oven over medium heat. Add the onion, chopped fennel bulb, and salt. Cook until the vegetables are softened and translucent, 5 to 7 minutes.

Stir in the garlic and thyme and cook until fragrant.

Stir in the flour and cook for 1 minute to toast the starches.

Pour in the wine, scraping up any browned bits from the bottom of the pot, and cook until the liquid has nearly evaporated, about 1 minute.

Stir in the broth, water, celery root, potatoes, apple, bay leaf, and orange zest. Bring the liquid to a boil over medium-high heat.

Reduce the heat to low, partially cover, and simmer gently until the stew is thickened and the vegetables are tender, 35 to 40 minutes.

Remove the pot from the heat and discard the bay leaf and orange zest strips.

Transfer two cups of the chowder and the heavy cream to a blender. Puree until completely smooth, about 1 minute, then return the mixture to the pot.

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