For the tots:
- 6 medium starchy potatoes, peeled and cubed
- 3 tablespoons finely grated onion
- 4 tablespoons flour
- 1 egg
- 1 tablespoon fresh rosemary, minced
- Finely ground black pepper
- Oil, for frying
For the burgers:
- 1 1/2 pounds beef tenderloin, cut into cubes, or ask your butcher to coarse-grind the tenderloin for you
- 1/4 cup parsley, very finely chopped
- 1/4 cup grated onion
- Kosher salt and pepper
- 1 large egg yolk, beaten
- 2 tablespoons butter
- Sliced mozzarella or mild cheddar (optional)
- Brioche burger rolls, split, lightly toasted
For the tots, boil the potatoes until tender. Drain and pass through a ricer into a mixing bowl. Combine with the grated onion, flour, egg and minced rosemary and season with salt and pepper. On a floured surface, roll into 1-inch-thick logs and cut into tots.
Heat the frying oil in a countertop fryer to 350°F or heat a few inches of oil in a large Dutch oven. Fry the tots in small batches when the burgers are ready to serve. Season the cooked tots with a little extra salt.
For the burgers, with the meat grinder attachment on a stand mixer, coarse-grind the meat if you didn't get it already ground. Transfer the ground meat to a mixing bowl and add the parsley, onion, salt, pepper and egg yolk. Form four patties.
Heat a cast iron pan or large heavy skillet over medium-high heat. Add the butter and, when it foams, add the burger patties. Cook for 6-8 minutes for rare to medium-rare doneness, turning once. Add the cheese in at the last minute for cheeseburgers, tenting with foil to melt.
Serve the burgers on brioche rolls with tots alongside.