Cook the polenta according to the package directions.
Heat oven to 400° F. Toss the bell peppers, tomatoes, fennel seeds, paprika, 2 tablespoons of the oil, and ¼ teaspoon each salt and black pepper on a rimmed baking sheet and roast until the vegetables are slightly charred, 14 to 15 minutes. Toss with the vinegar.
Meanwhile, heat the remaining tablespoon of oil in a large skillet over medium heat. Season the steak with ½ teaspoon salt and ¼ teaspoon black pepper. Cook, in 2 batches, until an instant-read thermometer inserted in the center registers 130° F, 2 to 5 minutes per side for medium-rare*. Let rest for 5 minutes. Slice against the grain.
Serve the polenta topped with the steak and vegetables.
*This recipe includes the Real Simple test kitchen’s preferred cooking temperatures (considered safe by many experts) for steaks and chops cooked to their juicy best. For maximum food safety, the U.S. Department of Agriculture recommends temperatures of 145° F for all beef, lamb, and pork steaks and chops.