Preheat the oven at 200°C and line a baking tray with greased foil.
Heat the oil and sauté the garlic for a minute.
Add the mushrooms and pepper and cook until slightly soft.
Add the Swiss chard, thyme and lemon zest and cook until wilted. Season and leave to cool slightly.
Make a lengthwise slit on the fish fillets (keeping both ends in tact) to make room for the stuffing.
Stuff half of the stuffing into each fillet and place in a baking sheet lined with foil.
Bake in oven for 10 minutes.
Add the sugar snaps and bake for a further 2 minutes of cooking until the fish flakes easily when tested with a fork. The fish should still be pink inside.
Transfer to serving platter and finish with a squeeze of lemon juice.