Preheat oven to 425 degrees. Line a baking sheet with foil or parchment paper.
Toss potatoes with garlic, oil, rosemary, sage, and a pinch of salt and pepper. Place potatoes on baking sheet and bake 20 minutes.
Rub cod with pinch of salt and pepper.
Remove baking sheet from the oven, move potatoes to make room for the cod, and place fish on the baking sheet next to the vegetables. Return to oven and cook another 15 minutes.
Remove from oven and spritz lemon juice on fish.