Heat oven to 200C/180C fan/gas 6. Wash the potatoes and place in a large pan of cold, salted water. Cover and bring to the boil, then simmer for 15 mins until cooked.
While the potatoes cook, combine the lemon zest, breadcrumbs, butter and peppercorns. Put the salmon fillets in a large roasting tin and sprinkle 1 tbsp of crumbs on each fillet. Pop the lemon wedges in the tin and bake for 8-10 mins until the salmon is cooked and the crumb is golden and crunchy.
When the potatoes are cooked, drain in a colander and crush lightly. Add a knob of butter, season and scatter with the thyme. Serve with broccoli or spring greens.