Cured salmon stuffed with latkes

Name: Cured salmon stuffed with latkes
Ingredients: 1kg wild salmon fillet, skin-on, fins and bones removed; 45g sea salt; 45g sugar; ¼ tsp fennel seeds, crushed; ¼ tsp coriander seeds, crushed; ¼ tsp pink peppercorns, crushed; ¼ tsp ground cinnamon; ½ tsp cumin seeds, crushed; 1 tbsp tarragon, chopped; 1 tbsp chives, chopped; 1 tbsp chervil, chopped (or use parsley as an alternative); 1 tbsp olive oil, plus extra to serve; soured cream and lemon wedges, to serve; 1 spring onion, thinly sliced; 2 tbsp parsley, finely chopped; 1 tbsp potato starch, or cornflour; 2 eggs; 1 starchy potato, such as a King Edward (around 400g), cut into matchsticks; 2 tbsp unsalted butter