1. Preheat the oven to 475°F [240°C].
2. In a large skillet over medium heat, add 3 tablespoons of the oil. Once the oil is hot, add the shallots and cook, stirring, until tender and golden, about 5 minutes. Add the red pepper flakes and cook for 30 seconds more. Remove the skillet from the heat and set aside to cool slightly. Stir in the pistou and the lemon zest from one of the lemons. Season with fine sea salt and pepper.
3. Pat the fish dry and season with fine sea salt and pepper, inside and out. Thinly slice one of the lemons; remove and discard the seeds. Stuff each fish cavity with 3 or 4 lemon slices, then tuck 1 to 1½ tablespoons of the shallot-pistou mixture into each cavity and arrange the fish in a single layer on a baking sheet. Drizzle each fish with a tablespoon of the remaining olive oil and place in the oven. Roast until the flesh is opaque and flaky, 15 to 20 minutes. Transfer to a serving platter, drizzle with olive oil, and sprinkle with flaky sea salt. Cut the remaining lemon into wedges and serve with the fish.

For the Pistou

If you are making the pistou with a mortar and pestle:

1. Place the garlic and salt in a mortar and grind until a paste forms. Add the basil leaves by the handful
   and pound them, scraping the sides of the mortar often, until they are almost smooth and all the basil is
   incorporated. Slowly add the oil, 1 tablespoon at a time, smashing until it’s all combined. Season with
   salt to taste.

If you are making the pistou with a blender:

1. Place the garlic cloves in the blender and pulse until roughly chopped. Add the salt and basil and pulse, scraping the sides of the blender often, until nearly smooth. With the blender running, slowly pour in the oil until it’s all combined. Season with salt. The pistou can be made and stored in an airtight container in the refrigerator for up to 3 days in
   advance.