(OPTIONAL) Brown the patties:
For extra flavor, quickly sear the hamburger patties in a skillet over medium-high heat for 2-3 minutes per side. Do this if you are using freshly made patties and not frozen. This step is optional and can be done with the frozen patties too but for 4-5 minutes. This will add depth to the dish.

Make the sauce:
In a large mixing bowl, combine the cream of mushroom soup, beef stock, onion soup mix, Worcestershire sauce, ketchup, minced garlic, salt, pepper, and beef bouillon. Whisk until well mixed. Then whisk in the flour slowly to avoid clumping.

Layer in the slow cooker:
Place the patties in the bottom of your 8-quart slow cooker. Layer sliced mushrooms in between the patties to avoid them sticking together.

Pour the sauce:
Pour the mixture evenly over the patties and mushrooms, making sure everything is well coated.

Cook:
On LOW for 6-8 hours or on HIGH for 3-4 hours. Make sure the patties are fully cooked and the ingredients have melded together.

(OPTIONAL) Adjust Sauce Thickness:
If you prefer a thicker gravy, mix 1 tbsp cornstarch with 2 tbsp of beef stock (or cold water) and stir it into the sauce about 30 minutes before serving. Cook on high uncovered to thicken. To thin out the sauce just stir in some more beef stock.