Heat a large skillet over medium heat and cook the ground beef, breaking it into small crumbles, until no pink remains and browned spots develop, about 8 to 10 minutes.

If there is a lot of grease in the skillet, spoon off most of it, leaving a thin coating behind with the beef.

Sprinkle the flour evenly over the hot browned beef and stir until every piece is coated and no dry flour remains.

Slowly pour in the whole milk while stirring and scraping up the browned bits from the bottom of the skillet, then add the kosher salt and several good grinds of black pepper.

Stir the beef and milk mixture over medium heat until it warms through and begins to thicken slightly, about 3 to 5 minutes.

Pour the beef and milk mixture into a 3- to 4-quart slow cooker, scraping every bit from the skillet into the cooker.

Cover and cook on LOW for 4 to 5 hours or on HIGH for 2 to 2 1/2 hours, until the gravy is creamy and lightly thickened, stirring once or twice during cooking if convenient.

Check the gravy near the end of cooking; if it is too thick, stir in a splash of milk, or if it is too thin, remove the lid, switch the slow cooker to HIGH, and cook for another 15 to 20 minutes, stirring occasionally. Taste and add more salt and black pepper if needed.

Shortly before serving, toast the white bread until golden brown and cut each slice into triangles. Arrange the toast points on warm plates, spoon the hot hamburger gravy over the top, finish with more black pepper, and serve steaming.