It's all about the relish. Use a mix of colored peppers for this almost-fruity condiment that is equally good with fish, chicken and pork.
For relish, in an extra-large, deep skillet, heat olive oil over medium. Add peppers and stir to coat with oil. Add 1 teaspoon salt. Cook until peppers are softened, about 10 minutes, stirring occasionally. Stir in garlic and crushed red pepper. Cook until fragrant, about 5 minutes. Add balsamic vinegar and sugar, reduce heat to prevent scorching, and cook until vinegar reduces and becomes syrupy, about 10 minutes. Add dried tomatoes and cook 2 to 3 minutes to incorporate flavors. Remove from heat and add basil. Set aside.
Pat steak dry and season both sides generously with salt and cracked black pepper.
Grill steaks on rack of a covered grill directly over medium 10 to 12 minutes for medium-rare (145°) or 12 to 15 minutes for medium (160°). Remove steaks from grill, tent with foil and let rest 5 to 10 minutes. Slice thinly against the grain. Serve relish warm or at room temperature over sliced flank steak.