Salisbury Steak Slow Cooker-Style

Mix ground beef chopped onion celery milk chopped mushrooms and panko together in a large bowl; shape into 10 patties.

Mix all-purpose and whole wheat flours together in a wide shallow bowl. Dredge patties in the flour mixture just to coat and move to a platter taking care to not stack the patties.

Heat 2 tablespoons oil in a large skillet over medium-high heat. Cook patties in hot oil until browned 3 to 5 minutes per side. Arrange browned patties in a slow cooker crock.

Heat remaining oil in a small skillet over medium heat. Cook and stir sliced mushrooms and sliced onion in hot oil until tender about 5 minutes.

Stir cooked mushroom and onion mixture with cream of mushroom soup beef broth Worcestershire sauce onion gravy mix and black pepper together in a bowl until smooth; pour over the beef patties in the slow cooker.

Cook on High until patties are very firm hot and grey in the center 3 to 3 1/2 hours (or on Low for 4 to 5 hours). An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).