Plunge the tomatoes in boiling water for 1-2 minutes. Cool briefly and remove the skins. Cut the peeled tomatoes into 1/2-inch slices. Transfer to large kettle. Add onion, red pepper, salt, celery seed, brown sugar, and vinegar. Cover and bring to a boil. Simmer over low heat for three hours. Or line a slow cooker with a cooking bag, transfer mixture to the cooker, set on low, and cook for 5 hours. Ladle into jars and refrigerate. The chili sauce will keep in the refrigerator for weeks.