Double Smash Burgers with Tangy Mustard Sauce

Steps

1. Prepare the toppings first: wash and tear the romaine, slice the tomato, and slice the onion as thinly as possible. Set them aside.
2. Make the sauce: stir the Greek yogurt, mustard and garlic powder together in a small bowl.
3. Divide the ground beef into 8 equal loose balls; do not pack them tightly. Keep them cold until cooking.
4. Heat a large cast-iron or heavy skillet over medium-high heat. Toast the whole-wheat bread slices in the dry skillet about 1 minute per side, then set aside on a plate.
5. Add the canola oil to the hot skillet. Place 4 beef balls in the pan, leaving space between them, and smash each one flat to about 1/4 inch thick with a sturdy spatula pressed down for 10 seconds.
6. Season the tops of those 4 patties with half the salt and half the black pepper. Cook 2 minutes until the edges are deeply browned, scrape under each patty, flip and cook 1 minute more, until they reach 160°F (71°C). Transfer to a plate.
7. Repeat with the remaining 4 beef balls, seasoning them with the remaining half of the salt and the remaining half of the black pepper, cooking to 160°F (71°C).
8. Assemble: spread the mustard sauce on 4 slices of toasted bread, then layer the romaine, two patties, the tomato and the onion. Top with the remaining 4 slices of toasted bread and serve right away.