Season fish on both sides with salt and pepper.
Place flour in a zip-lock bag. Add 1 piece of fish at a time to bag and toss well to evenly coat in flour. Repeat process with remaining 3 pieces of fish.
Heat oil and butter in a large non-stick pan over medium-high heat. Add fish fillets and sear for 30 seconds on each side.
Add spring onion, tomato, mussels and wine. Stir to combine then bring to a simmer and cook for 2 minutes.
Stir in stock, zest, Moroccan seasoning (if using) and lemon slices. Bring back to a simmer then cover pan with lid and cook for 6 minutes.
Discard any mussels that don’t open before serving.