Preheat oven to 425°F (220°C). Toss potatoes with 1 tbsp (15 mL) oil on foil-lined rimmed baking sheet. Sprinkle with 1/4 tsp (1 mL) salt and pinch of pepper. Bake potatoes until almost tender and starting to turn golden, about 20 min.
Pat fish dry with paper towel. Brush fish with remaining oil, and sprinkle with lemon zest and remaining salt and pepper.
Push potatoes to sides of baking sheet and place fish in middle of baking sheet. Scatter green beans around the fish.
Roast until salmon is just cooked through and green beans are tender, about 10 min. Serve with lemon wedges.
Use four skinless tilapia fillets instead of salmon.