Prep and portion the beef. Divide the 1 lb (454 g) 80/20 ground beef into four equal portions (about 4 oz / 113 g each). Handle the meat as little as possible when forming patties—overworking compacts the proteins and produces a tough burger. Gently form slightly loose balls and flatten them into discs about 3⁄4-inch thick; they’ll shrink while cooking.

Season just before cooking. Sprinkle each patty with a light pinch of kosher salt (about 1/4 tsp per patty) and a little freshly ground black pepper. Salting too early draws moisture; salting right before cooking seasons the surface and helps form a better crust.

Caramelize the onion. Thinly slice the large yellow onion. In a medium skillet over medium-low heat, melt 1 tablespoon butter with 1/2 tablespoon oil. Add the onion and a pinch of salt. Cook slowly, stirring occasionally, until the onions turn deep golden-brown and sweet, about 18–22 minutes. Slow cooking converts onion starches to sugars—that sweet, jammy flavor is what elevates the burger. If they begin to stick, add a splash of water and lower the heat; you want deep color, not ashes.

Make the garlic-herb butter and spread. Mince 1 clove garlic and finely chop the parsley. In a small bowl, mix 2 tablespoons softened butter with the garlic and chopped parsley (optional). This butter will be used to toast the buns and adds a glossy, savory finish. Keep the remaining 1 tablespoon butter for searing the burgers if desired.

To toast the buns. Halve the brioche buns and brush the cut sides with the garlic-herb butter. Toast them in a hot skillet (or under a broiler) for 30–60 seconds until golden and just crisp. Toasting creates a flavor contrast and a barrier that helps prevent sogginess from juices and condiments.

Preheat the pan. Heat a heavy skillet or cast-iron pan over medium-high heat for 3–4 minutes until hot but not smoking. Add the 1 tablespoon oil and swirl to coat; a properly preheated pan ensures immediate contact heat to form a brown, flavorful crust (Maillard reaction).

Sear the patties. Place the patties in the hot pan leaving space between them. Resist moving them for the first 2–3 minutes so a brown crust can form; this crust locks in flavor and builds texture. After 2–3 minutes, flip once using a spatula. If you like a smash-style thinner burger, press down firmly for the first 10–15 seconds to maximize contact; for a thicker patty, do not press after initial sear.

Finish cooking and melt the cheese. After flipping, cook another 2–3 minutes for medium (adjust time for thickness and desired doneness). Place a slice of cheddar on each patty for the last 1 minute and cover the pan briefly to melt the cheese evenly. Covering traps steam which helps the cheese melt without overcooking the meat.

Build the burger. Spread 1 tablespoon mayonnaise on the bottom halves of the toasted buns, then a light smear of Dijon on the top half. Place the patty on the bottom bun, top with a generous spoonful of caramelized onions, 2 slices of dill pickle, then the top bun. The layers provide creamy, tangy, sweet, and salty contrasts in every bite.

Rest briefly and serve. Let assembled burgers rest 1–2 minutes before serving to allow hot juices to settle and the cheese to set slightly. Serve immediately for best texture and contrast between crunchy-tender bun and juicy patty.