DIRECTIONS
Cook the Potatoes:
Preheat oven to 450°F with a rack in the upper and lower third of the oven. Place a sturdy baking sheet in the oven while it preheats. In a medium bowl, toss the potatoes with 1½ tablespoons oil, oregano, and a pinch of salt and pepper. Spread on the hot baking sheet and roast on the lower rack until tender and lightly browned, 15 to 20 minutes.
Make the dressing:
In the same bowl, combine the lemon zest and juice, garlic, and 2 tablespoons oil. Season with salt and pepper.
Cook the Fish:
Remove the pan with the potatoes from the oven and switch the oven to broil. Season the fish with salt and pepper and drizzle lightly with oil. Move some of the potatoes to make space and add the fish and olives to the baking sheet in the empty spaces. Transfer to the upper rack, and broil until fish flakes easily around the edges, 3 to 5 minutes depending on the thickness. Transfer the fish to plates.
Finish and Serve:
Add the spinach to the potatoes, toss with half of the dressing, and broil just until wilted, about 30 seconds. Serve alongside the fish and drizzle with remaining dressing. Sprinkle with feta and mint.