In a large stainless steel skillet, heat oil over medium heat until shimmering. Add whole serrano and cook, turning occasionally, until spotty brown on all sides, 3 to 5 minutes. Transfer serrano to plate and set aside. Allow skillet to cool down slightly, about 2 minutes.
Add onion to skillet with oil. Cook over medium heat, stirring occasionally, until onions are translucent, about 5 minutes. Add garlic, chopped serrano, and black pepper, cook, stirring constantly, until garlic just begins to turn golden, about 1 minute. Stir in tomatoes and salt. Add reserved whole serrano, and bring tomato mixture to a simmer. Lower heat to medium-low, cook, uncovered, stirring occasionally —especially toward the end—scraping the bottom of the pan to prevent scorching, until tomatoes have completely broken down and sauce has thickened into a jammy consistency, 1 ½ to 1 ¾ hours. Adjust heat to maintain a vigorous, steady simmer—bubbles should be active but not rolling like a full boil.
Off heat, stir in pomegranate molasses, if using. Serve with warm pita bread.
