1 yellow bell pepper quartered lengthwise and thinly sliced crosswise (ribs and seeds removed)
2 garlic cloves thinly sliced
1/4 teaspoon dried oregano
3 TB red-wine vinegar
2 TB tomato paste
In a small skillet, heat 2 teaspoons olive oil over medium heat. Trim and season 1 shell steak (6 to 8 ounces) with coarse salt and ground pepper; cook until desired doneness, about 2 minutes per side for medium-rare. Transfer steak to a plate; cover with foil to keep warm.
Make sauce: Add 1 halved and thinly sliced small onion to pan; cook, stirring often, until starting to soften, about 3 minutes.
Add 1 yellow bell pepper, quartered lengthwise and thinly sliced crosswise (ribs and seeds removed), 2 thinly sliced garlic cloves, and 1/4 teaspoon dried oregano; season with salt and pepper. Cook until bell pepper is crisp-tender, about 5 minutes.
Add 3 tablespoons red-wine vinegar, and bring to a boil; continue boiling until almost evaporated, about 1 minute. Add 2 tablespoons tomato paste and 1/2 cup water; cook 1 minute more. Spoon pepper sauce over steak.