Spice-Crusted Salmon with Sautéed Potatoes & Spinach

1 tsp fennel seeds 1 tsp coriander seeds 1 tsp yellow mustard seeds 6 wild salmon fillets , skin on (about 140-175g/5-6oz each) 3-4 tbsp olive oil FOR THE SAFFRON BEURRE BLANC 3 shallots , finely chopped 100ml white wine vinegar 150ml fish stock 1 pinch saffron (no...