Open packages of minced meat and divide into smaller clumps. Place the clumps into a large bowl, distributing the beef and pork into more-or less even proportions.
Add the bread crumbs. Set aside the bowl.
In another bowl add all other ingredients except the sweet pickle relish. Mix it all together with an electric beater or in a blender.
Pour the mixed seasonings on the meat and breadcrumbs. Add sweet pickle relish.
With gloved hands, work in the seasonings into the meat and breadcrumbs until well incorporated and everything looks mixed evenly.
Cover bowl with plastic wrap and place in fridge. Length of time depends on the amount of meat used. If it's the minimum 3kg, 4-6 hours of marinate/sit time; more than that will need overnight at least to allow the meat's flavour to absorb into the meat.
Prepare baking trays: line trays with baking paper. If choosing to add butter cubes, start cubing butter.
Set out your workarea with the bowl of meat and trays.
Take into gloved hands the amount of meat mix you want for your patty. If you want butter in the middle, place a cube in the middle of your ball of meat, reform the ball and press slightly into patty shape.
Place patties on the prepared trays. If wished, place another layer of baking paper on top and another layer of patties; no more than two layers of patties. Place another layer of baking paper on top to protect the patties and put in freezer, if freezing.
Freeze for at least 8 hours, before removing burger patties to zip lock bagged portions.
Use burger patties as you wish. Recipe is also good for meatballs (freeze in the same way.)