3 tbsp. extra-virgin olive oil
4 Fresh rosemary springs
1 tsp. kosher salt, divided
680 gram. ground chuck
1/2 tsp. Freshly ground black pepper
115 gram blue cheese (about 100 gram, crumbled)
4 Hamburger buns
8 Buttered lettuced leaves
In a large skillet over a medium heat, heat oil. Add the rosemary sprigs and fry for about 1 minute, or until the leaves are crispy. Use tongs to pluck the sprigs out of the oil and place on a paper towel. When cool enough to handle, pull the leaves from the sprigs.
Add the onions and 1/4 teaspoon of the salt to the skillet and stir to coat. Cover and let cook, undisturbed, for 7 to 8 minutes, until they start to soften. Remove the lid and continue to cook until softened and caramelized, stirring occasionally, 7 to 8 minutes more. Remove from the heat and stir in the rosemary leaves.
Preheat the oven (with the oven rack in the middle) to 200°C. Shape the beef into 4 patties about 3/4″ thick. Sprinkle both sides with pepper and the remaining 3/4 teaspoon salt.
Heat a large skillet, preferably a cast-iron skillet, over medium heat. Add the patties and cook for 5 to 6 minutes, until the undersides have a nice dark brown crust. Flip the patties and cook for 5 to 6 minutes more for medium doneness. (Sorry about the splatters!!)
Transfer the burgers to a rimmed sheet pan. Dividing evenly, crumble the blue cheese over each burger. Bake until the cheese melts, 2 to 3 minutes. Warm your buns at the same time.
Build your burgers with the buns, patties, lettuce, caramelized onions and rosemary.