How to Make It
In a stand mixer fitted with the paddle, combine the ground meats, powdered milk, spices and 1 tablespoon of salt. Mix at low speed until blended. Add the egg and cream and mix until incorporated. Mix in the cheese.
Heat an oiled cast-iron griddle or grill pan. Shape the meat into 6 oval 1/2-inch-thick patties. Cook over moderate heat, turning once, until well browned and cooked through, 8 minutes. Transfer to a plate; cover loosely with foil.
Add the onion and a large pinch each of salt and pepper to the griddle. Cook over moderately high heat, stirring, until golden and softened, 5 minutes. Transfer to a plate.
Butter the bread on both sides. Griddle over moderate heat for 2 minutes, turning once. Spread with mustard. Arrange the patties on half the toasts, top with the griddled onions and pickles and serve.