Skin and flake the fish. Cook the potatoes in a saucepan of salted boiling water. Drain well and mash. Mix in the fish, coriander and egg, and flavour with salt and pepper and a good squeeze of lemon or lime juice. Shape into patties, flattening them between your palms. Coat with toasted crumbs.
Cut the limes in half, brush generously with oil and grill in a frying pan until lightly charred and smoky.
Mix the spring onions, tomatoes and coriander, and season with salt and pepper. Add a little olive oil and lime juice. Spoon into a bowl.
Heat oil in a medium frying pan for shallow-frying, and fry the frikkadels until crisp and golden; 2 or 3 minutes on each side should do it.
Serve hot or at room temperature, never chilled, as they lose all their flavour. Rougail and charred limes can be presented alongside or in separate bowls.