Preheat the oven to 475°F. Take the fish out of the fridge 15 minutes before you want to use it. When the oven is hot, place the pieces of fish slightly apart on a small parchment-lined rimmed baking sheet. Season with salt and pepper, and rub all over with the pesto sauce. Bake until the fish is opaque and flaky, about 12 minutes.
Meanwhile, make the spinach. In a wide skillet, heat the olive oil over medium heat. Add the garlic, and cook about 30 seconds. You don’t want it to brown. Add the spinach; it will look like too much for the pan, but it will wilt down. Season with salt and pepper, and stir with tongs until the spinach has just wilted, about 2 minutes.
Make a bed of the garlicky spinach on a serving dish, and use a spatula to place the fish on top. Sometimes, I pour a little of the basil oil that bakes off the fish to the spinach. Yum.
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