This is the recipe for the peppers and onions that I served alongside that Pan-Roasted Pork Tenderloin. It is from The New Doubleday Cookbook by Jean Anderson and Elaine Hanna. This is my go-to cookbook for any basic recipe. It is utterly reliable. The addition of red wine, tomato paste, and oregano are something I picked up somewhere else but can't remember where.
Italian Peppers and Onions
2 tablespoons olive oil
3 large sweet red or green pappers or 8 Italian sweet green peppers, washed, cored, seeded, and cut in long, thin strips
1 medium-size Spanish onion, peeled and sliced thin
1 clove garlic, peeled and crushed
1/2 teaspoon salt
2 tablespoons tomato paste
3 tablespoons red wine
1 teaspoon oregano
Heat oil in a large, heavy skillet over moderate heat. Add peppers, onion, and garlic and stir-fry 8 minutes until onion is golden. Add all other ingredients, cover, turn heat to low, and simmer 5 minutes, shaking pan occasionally. Uncover, cook, stirring, 1-2 minutes to drive off excess moisture (I don't usually find this necessary because of the tomato paste); serve.