Preheat the grill to high heat.
In a large bowl, mash the bread and milk together with a fork until it forms a chunky paste. Add the salt, pepper, garlic, Worcestershire sauce, and ketchup and mix well.
Add the ground beef and scallions and break the meat up with your hands. Mix everything together until evenly combined. Divide the mixture into 8 equal portions and form compact balls. Flatten the balls into 3/4-inch patties about 4-1/2 inches across. Form a slight depression in the center of each patty to prevent the burgers from puffing up on the grill.
Oil the grilling grates. Grill the burgers, covered, until nicely browned on the first side, 2 to 4 minutes. Flip burgers and continue cooking for a few minutes more until the desired doneness is reached. Before serving, toast the buns on the cooler side of the grill if desired.
Freezer-Friendly Instructions: The uncooked burgers can be frozen for up to three months. (Freeze the burgers on a baking sheet or plate so their shape sets, then transfer them to a sealable plastic bag for easy storage.) Defrost the burgers overnight in the refrigerator prior to serving and then cook as directed.