This sweet-and-sour sauce is good with steak, but it can also be served with chicken, pork, or fish.
In a small skillet, heat olive oil over medium heat. Trim and season steak 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 onion to pan; cook, stirring often, until starting to soften, about 3 minutes.
Add bell pepper, garlic, and dried oregano; season with salt and pepper. Cook until bell pepper is crisp-tender, about 5 minutes.
Add red-wine vinegar, and bring to a boil; continue boiling until almost evaporated, about 1 minute. Add tomato paste and 1/3 cup water; cook 1 minute more. Spoon pepper sauce over steak.
If you just want to make the sauce, omit step 1. Then heat 2 teaspoons oil before proceeding with steps 2 through 4. Chill, and save as a condiment for cold cuts and sandwiches.