Place the potatoes in a bowl. Toss them with the olive oil, lemon zest, thyme, salt and pepper until each potato is coated. Spread them on a baking sheet in a single layer.
Roast the potatoes for 20 minutes, or until they begin to get golden and caramely (larger potatoes may take a bit longer - 5 or 10 minutes more). Remove the pan from the oven and use a spatula to push the potatoes to the edges.
Place the salmon (if it has skin, place it skin-side down) on the baking sheet. Sprinkle with salt and pepper. Cover the top with the pesto. Place the sheet back in the oven and roast for 12 to 15 minutes, or until the salmon is opaque and flakes with a fork.