Toss the diced baby potatoes with 1 tbsp of olive oil, 1/2 tsp of salt, and 1/4 tsp of pepper. Spread them evenly on a baking sheet and roast for 10 minutes.
Pat the 4 fish fillets dry with paper towels. Rub them with the remaining 1 tbsp of olive oil, 1/2 tsp of salt, 1/4 tsp of pepper, and 1 tbsp of chopped thyme.
Remove the baking sheet from the oven. Carefully push the partially cooked potatoes to the sides and place the seasoned fillets in the center. Top each fillet with a lemon slice.
Return the pan to the oven and cook for 10-12 minutes until the fish is opaque and flakes easily with a fork.