1/4 teaspoon canola oil
5 ounces freshly ground beef, divided into two 2 1/2-ounce balls
Kosher salt and freshly ground black pepper
1 medium onion, thinly sliced (about 1 cup sliced onions)
2 slices American cheese
2 soft hamburger buns
Mustard, pickles, and other condiments, if desired
-Spread canola oil inside a large cast iron skillet with a paper towel. Heat over high heat until lightly smoking. Add balls of beef and press down with the back side of a spatula until completely flattened. Season generously with salt and pepper.
-Working quickly, spread onions evenly over cooking burgers and press down to embed onions into meat. Let cook with no further pressing until burgers are well browned, about 2 minutes.
-Carefully flip burgers with a stiff spatula, making sure to pick up all browned meat from skillet. Let cook until onions start to soften, about 1 minute. Add cheese and top with top bun. Place bottom bun upside-down over top bun to allow it to steam in the onion vapors.
-When buns are soft, transfer bottom buns to a plate. Add condiments if desired. Lift up onions, burger, and top bun together and transfer to bottom bun. Repeat with second burger. Serve immediately.