Cook the polenta according to the package directions.
Meanwhile, heat the oil in a large skillet over medium-high heat. Season the steak with ½ teaspoon salt and ¼ teaspoon black pepper and cook, 3 to 5 minutes per side for medium-rare. Let rest for 5 minutes before slicing.
Add the bell peppers, shallot, and ¼ teaspoon each salt and black pepper to the drippings in the skillet. Cook, tossing frequently, until beginning to soften, 3 to 5 minutes. Add the vinegar and ¼ cup water and cook, tossing, until the vegetables are tender and the liquid is almost evaporated, 2 to 3 minutes. Add the spinach and olives and cook, tossing, until the spinach begins to wilt, 1 to 2 minutes more.