Cook polenta according to the package directions.
Slice bell peppers and shallot.
In a large skillet heat oil over med-high heat. Season steak with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Cook steak 3-5 minutes per side for medium-rare. Let rest for 5 minutes before slicing.
Add peppers, shallot, and 1/4 teaspoon each salt and pepper to the drippings in the skillet. Cook, tossing frequently, until beginning to soften, 3-5 minutes.
Add the vinegar and 1/4 cup water and cook, tossing until vegetable are tender and the liquid is almost evaporated, 2-3 minutes. Add the spinach and cook, tossing, until spinach begins to wilt.
Serve steak and vegetables with the polenta.