Milksteak is a meal enjoyed by Charlie Kelly. It was first mentioned in the Season 5 episode "The Waitress is Getting Married" where Charlie wants to put it down as his favorite food in an online dating profile. It is later mentioned in the same episode as Charlie orders it in the restaurant.
It was mentioned again in the Season 13 episode "Charlie's Home Alone" as one of the brown foods Charlie eats as part of his color rituals to make the Eagles win.
The "milksteak" Charlie refers to is a literal steak boiled in milk. He orders it "boiled over hard, with a side of your finest Jelly Beans, served raw."