In a large pot, heat the olive oil over medium heat. Add the fennel slices, onion, and garlic, cooking until the vegetables are soft but not browned, about 5 minutes.
Stir in the red pepper flakes and cook for an additional minute to release the flavors.
Pour in the white wine and let it simmer until the liquid is reduced by half, which should take around 3-4 minutes.
Add the fish or vegetable stock and diced tomatoes with their juices to the pot. Season with salt and black pepper.
Bring the mixture to a gentle boil, then reduce the heat to low and simmer uncovered for 15 minutes, allowing the flavors to meld together.
Rinse the clams under cold running water, discarding any that are open and do not close when tapped.
Add the clams to the pot, cover with a lid, and cook over medium heat for 6-8 minutes or until the clams have opened. Discard any clams that remain closed.
Remove from heat, and stir in the chopped parsley, lemon zest, and lemon juice.
Taste and adjust seasoning if necessary.
Serve the stew hot, with crusty bread for dipping into the flavorful broth.