Preheat the oven to 200 degrees C/400 degrees F. Line a baking sheet with greased foil.
Heat the oil in a saute pan over medium-high heat. Add the garlic, saute for 1 minute, then add the mushrooms and peppers and cook until slightly soft. Add the Swiss chard, thyme and lemon zest and cook until wilted. Season with salt and pepper. Leave to cool slightly.
Make a horizontal slit in the thickest part of the salmon fillets (keeping both ends intact) to make room for the stuffing.
Stuff the fillets with the cooled stuffing and place on the prepared baking sheet.
Bake for 10 minutes. Add the sugar snaps and bake for a further 2 minutes, or until the fish flakes easily when tested with a fork--the fish should still be pink inside.
Transfer to a serving platter and finish with a squeeze of lemon juice.