Heat oven to 400° F.
On a large rimmed baking sheet, toss the potatoes, lemon, thyme, 2 tablespoons oil, and ¼ teaspoon each salt and pepper. Arrange in a single layer.
Roast, tossing once, until the potatoes begin to soften, about 20 minutes.
Toss the olives with the potato mixture; nestle the fish in it. Drizzle the fish with the remaining oil and season with paprika and ¼ teaspoon each salt and pepper.
Continue to roast until the potatoes are golden brown and crisp and the fish is opaque throughout, 12 to 15 minutes.