How to Make It
In a saucepan, over medium-low heat, simmer the shallots in ½ cup of the oil until they are light golden brown, about 12 minutes.
Transfer the shallots to a bowl and stir in the capers, vinegar, and ¼ teaspoon each salt and pepper. Set aside.
Heat 1 tablespoon of the oil in a large skillet over medium-high heat.
Season the fish with ¼ teaspoon each salt and pepper and cook until opaque, about 4 minutes per side. Divide among plates.
Wipe out the skillet and heat the remaining 2 tablespoons oil over medium heat. Add the spinach, ½ teaspoon salt, and ¼ teaspoon pepper.
Cook, stirring, until wilted, 2 to 3 minutes. Serve with the striped bass, drizzling the vinaigrette over the top.