Prep: 20 minutes
Total: 45 minutes
for the béchamel:
1 tablespoons unsalted butter
3/4 tablespoons all-purpose flour
3/4 cup milk
¼ cup cream
1 bay leaf
dash of red wine vinegar
kosher salt and freshly ground black pepper, to taste
pinch of nutmeg for the onions:
3 tablespoons unsalted butter
1 onion, sliced
kosher salt to taste for the burger:
20 ounces Dry aged, grass fed steak, sirloin, NY Strip, or Ribeye or 20 ounces ground beef, preferably 25% fat
kosher salt, to taste
8 slices sourdough rye loaf bread
4 tablespoons unsalted butter, at room temperature
4 tablespoons Chinese hot or Coleman's English mustard
¾ cup grated mixed milk or sharp white cheddar cheese
1. Set steaks aside in the freezer to firm up for chopping (if hand cutting your own burger).
2. Make the béchamel: In a small pot over medium heat, melt butter, whisk in flour, and cook, whisking, until golden brown. Stir in milk and cream and keep whisking until it boils and starts to thicken. Add the bay leaf and cook for 4 to 5 minutes longer.
3. Take pot off the heat, stir in a dash of vinegar and a pinch each of salt, pepper, and nutmeg and pour into a bowl to cool.
4. Caramelize the onions: Set a frying pan over medium heat and add the butter to melt. Add the onions, reduce the heat to medium-low, and cook, stirring occasionally, until the onions turn golden brown, 30 to 45 minutes. Season with salt.
5. Remove the steak from freezer (it should be firm but not frozen). Slice into 1/8-inch cubes.
6. Press the patties: Divide the beef into four 5-ounce balls and press with your hands into ¼- to ½-inch-thick patties, ideally using a 3 1/2-inch ring mold or tart mold. Just before you're ready to cook, season each patty on both sides with salt.
7. Put it all together: Preheat the broiler. Butter the bread slices and place buttered side down on a baking sheet. Spread mustard on the unbuttered sides and top four of the slices with grated cheese. Put baking sheet under the broiler until the cheese melts, about 3 minutes if the broiler is hot. Make sure there is even coverage of cheese so exposed bread edges don't burn.
8. Meanwhile, heat a cast-iron skillet over high heat. Add patties (but place a tablespoon of butter under each patty before placing in the skillet) and cook until deeply browned on one side, about 2 minutes. Flip and cook for another minute. Should be browned on one side, but not cooked beyond medium rare/medium Set each patty on a slice of bread, top with caramelized onions and béchamel, and make a sandwich with remaining slices of bread.
9. To finish: Wipe skillet with a paper towel or cook in the beef fat, reduce heat to medium, and toast sandwiches on one side, about 2 minutes. Flip, then toast the other side, and serve.