Meanwhile, in saucepan, melt 2 tbsp (30 mL) butter over medium heat; fry onion until softened, about 4 minutes. Add peas, 1 cup (250 mL) of the poaching liquid for fresh peas or 3/4 cup (175 mL) for frozen peas, 1/4 tsp (1 mL) salt and pinch pepper; bring to boil. Cover, reduce heat and simmer until peas are hot and tender, 6 to 10 minutes for fresh peas, 3 minutes for frozen. Add mint and cook for 1 minute. Transfer to blender. Squeeze 2 tsp (10 mL) lemon juice from remaining lemon; add to blender. Purée until smooth, adding more poaching liquid if necessary, until desired consistency.

Spoon pea purée onto plates and top with salmon. Top each fillet with slice of lemon from poaching liquid.