1. Clean the clams by soaking them in cold water with flour for at least 30 minutes to help release any sand, then scrub under cold water to remove debris.
2. Trim the fennel bulb, slice it vertically, remove the core, and slice thinly.
3. In a large heavy pot, melt the butter with olive oil over medium heat. Add garlic, shallots, and fennel; sauté until tender and fragrant, about 5 to 7 minutes.
4. Add the white wine, clam broth, and thyme to the pot; bring to a boil, then reduce to a simmer. Cover and simmer for about 15 minutes to blend the flavors.
5. Introduce the cleaned clams to the pot, cover, and cook until the clams open, about 10 minutes. Discard any clams that do not open.
6. Transfer the clams to a warm bowl. Season the broth with salt and pepper as needed before pouring over the clams.
7. Garnish with fennel fronds, thyme sprigs, and a drizzle of extra virgin olive oil before serving.