Heat the olive oil in a large pot over medium heat. Add the onion, garlic, and fennel and cook for 5 minutes until the vegetables soften.
Pour in the white wine and let it simmer for 3 minutes to reduce slightly.
Add the fish stock, diced tomatoes, thyme sprigs, salt, and pepper. Bring the mixture to a gentle boil, then lower the heat and simmer for 10 minutes.
Add the white fish pieces and cook for 4 minutes. Then add the shrimp and continue cooking for 3 to 4 minutes until both the fish and shrimp turn opaque.
Remove the thyme sprigs. Ladle the stew into bowls and top each serving with chopped parsley. Serve with lemon wedges on the side.