How to Make It
Cook the pancetta in a nonstick skillet over medium, stirring occasionally, until crisp, about 5 minutes. Transfer the pancetta and drippings to a 5-quart slow cooker. Add the onion, tomatoes, vinegar, and honey, stirring to combine. Partially cover, and cook on HIGH until the mixture is syrupy and jam-like, about 4 hours.
Stir together the thyme and black pepper in a small bowl. Sprinkle over the cod fillets. Place the fish on the tomato mixture in the slow cooker; cover completely, and cook on LOW until the fish flakes easily with a fork, about 25 minutes. Serve the cod with the tomato jam; sprinkle with the parsley, if desired.
Typically, fully covering the slow cooker during cooking is necessary since the lid locks in moisture and keeps the food from drying out. However, when making the jam, letting the moisture out by only partially covering the cooker is what gives the mixture its thick, syrup-like consistency.