Place the potatoes in large baking dish, pour 2 tablespoons of the olive oil over them, add most of the rosemary & parsley (saving some for later), all the garlic cloves, and salt, and pepper. Toss thoroughly. Put the dish in the preheated oven. After 20 minutes, turn the potatoes over and put back in the oven. Cook until the potatoes feel tender when tested with a fork, another 15 minutes or so.
Remove the dish from the oven. Push the potatoes to the sides, making room in the center for the fish fillets to lie flat.
Rinse and pat dry the fish fillets, laying them flat in the dish. Scatter the remaining parsley & rosemary over the fish, add salt and pepper, and the remaining 1 tablespoon of olive oil over the fillet. Return the dish to the oven and bake for 10-15 minutes, depending on how thick your fillets are.