Preheat oven to 350 degrees F. Lightly spray a shallow 1.5 quart baking dish or a 11 x 7-inch baking dish.
Using a food processor, Ninja or your hands, process sliced bread into small crumbs to make soft breadcrumbs. Place in a large bowl.
Add the beaten egg, 2 tbs milk and 2 tbs of the condensed soup; stir well. Add the ground beef, onion, salt, pepper, garlic powder and thyme if using. Mix well. Shape into 4 "football" shaped patties about 1-inch thick; place in prepared pan.
In a separate bowl, combine remaining soup concentrate, mushrooms, Gravy Master if using (this is merely for a richer color and not necessary for any flavor) and 1/4 cup of the milk. Stir until smooth. There is a range for the milk--the 1/4 cup is for thicker gravy, the 1/2 cup is for thinner gravy or if you just want more gravy (but it'll be thinner). Feel free to use more or less, but always start out with the lower amount.
Pour the soup mixture evenly over the patties in the pan. Cover either with a lid or aluminum foil. I used a Corning Ware pan that had a lid, but foil will work just fine.
Bake at 350 degrees F for about 30 minutes, or until patties are cooked all the way through and have an internal temperature of 160 degrees F.
Serve with mashed potatoes or buttered noodles.