Heat the oven to 425°F and coat a 9-by-13-inch baking dish with cooking spray.
Toss the potatoes with 2 tablespoons olive oil, 1/2 teaspoon salt and half the pepper in the dish and turn them cut-side down.
Roast the potatoes for 20 to 25 minutes, until their cut sides are golden and a knife meets only slight resistance.
Toss the green beans with the remaining oil and nestle them around the potatoes.
Season the salmon with the remaining salt and pepper, set the fillets on top and lay the lemon slices over the fish.
Bake for 12 to 15 minutes, until the salmon flakes at the thickest point and reaches 145°F and the beans are crisp-tender.
Rest the salmon for 3 minutes before serving.