How to Make It
Combine the tomatoes, scallions, vinegar, 1 tablespoon of the oil, and ¼ teaspoon each salt and pepper in a medium bowl.
Heat 1 tablespoon of the remaining oil in a large nonstick skillet over medium-high heat. Add the polenta and cook until golden and crispy, 4 to 5 minutes per side. Transfer to a plate; reserve the skillet. Add 1 more tablespoon of the oil to the reserved skillet. Season the cod with ¼ teaspoon each salt and pepper and cook until opaque throughout, 3 to 4 minutes per side.
Heat the remaining tablespoon of oil in a large skillet over medium-high heat. Add the garlic and cook until fragrant, 30 seconds. Add as much spinach as will fit in the skillet. Season with ¼ teaspoon each salt and pepper. Cook, tossing frequently and adding more spinach as there is room, until just wilted, 2 to 3 minutes.
Serve the spinach, cod, and polenta topped with the marinated tomatoes.