Add onion mixture to skillet with pork juices; cook 5 to 8 minutes, stirring frequently, until beginning to brown. Add red wine; cook 1 to 2 minutes, stirring frequently, until all liquid is absorbed. Stir in tomatoes; cook 3 to 5 minutes, stirring constantly, until mixture thickens. Pour mixture over pork in slow cooker. Add stock and anchovies; stir liquid. Cook on Low heat setting 8 hours or High heat setting 4 hours.