Using a sharp knife cut away any brown bits from the salmon fillet, then finely chop. Place in a bowl, then stir in the spring onions, mustard and sesame seeds. Season to taste. Divide into 4 portions, then using slightly wetted hands, shape into patties. Dust the patties in the flour, shaking off any excess. Place in the fridge for 10 minutes to firm up.
Meanwhile, make the roasted red pepper salsa. Place the red peppers in a bowl and stir in the tomatoes, lemon rind, sweet chilli sauce, oil and herbs. Season to taste and set aside at room temperature to allow the flavours to develop.
Heat a large non-stick frying pan over a medium heat. Add the olive oil to the frying pan and then add the salmon burgers. Cook for 3-4 minutes on each side, until lightly golden but still slightly pink in the centre. Be careful not to overcook them or they will quickly become dry. Set aside.
Meanwhile, heat a griddle pan until smoking hot. Add the slices of sourdough bread and chargrill for 1-2 minutes on each side, until nicely marked. Arrange on warmed plates and top with Little Gem lettuce leaves and the burgers. Spoon over the roasted red pepper salsa, then scatter over the coriander sprigs and serve with lime wedges.