Place butter, olive oil, leeks, bay leaves, wine and 1/4 tsp (1 mL) salt in a skillet over medium heat. Cover and bring to a simmer until tender, stirring occasionally, 45 to 50 min., taking care not to scorch. Remove from heat, discard bay leaves, transfer leek mixture to a bowl to cool, then refrigerate for 15 min. Preheat oven to 350°F (180°C).
Place salmon, skin side down, on a parchment paper-lined rimmed baking sheet. Season with remaining salt and spread cooled leek mixture over salmon. Bake on middle rack of oven, 20 min. or until salmon is cooked through and flakes easily.
Meanwhile, prepare lemon sour cream. Mix together sour cream, lemon zest and juice and a pinch of salt. Sprinkle salmon with dill and cut into 8 equal portions and serve with lemon sour cream.
Baking salmon with its skin on adds incredible flavour and moistness. Substitute 2 bunches of green onions for the leeks and simmer uncovered with the other topping ingredients, 10 to 12 min. Omit the dill and garnish with 1 tbsp (15 mL) chopped thyme.