Mix the mayonnaise, ketchup, mustard, finely diced pickles, pickle juice, garlic powder, smoked paprika, and black pepper until smooth but still slightly textured from the pickles. Taste and adjust with more pickle juice if it needs acidity. The sauce should be creamy, tangy, and thick enough to spread on the bread without running. The diced pickles left slightly textured rather than fully blended in give the sauce a bit of bite alongside its creaminess, which is part of what distinguishes it from a simple, uniform spread.

Heat the butter and oil in a wide skillet over medium heat. Add the sliced onions and salt. Cook for 25–35 minutes, stirring often, until deeply softened, golden-brown, glossy, and jammy. If the onions start sticking or browning too fast before they soften, add 10ml water and scrape the pan. Add Worcestershire sauce at the end if using. Do not rush this over high heat. Burnt onions are bitter trash. Proper patty melt onions should be sweet, soft, and browned, not blackened and dry. The combination of butter and oil matters here — oil raises the effective cooking temperature just enough that the butter doesn't burn during this extended cook, while the butter still contributes richer flavour and colour than oil alone would.

Divide the beef into 4 portions of 100g each. Shape into thin patties slightly larger than the bread slices because they will shrink during cooking. Season both sides with kosher salt and black pepper right before cooking. Do not make thick burger patties here. Patty melts need thin patties so the sandwich griddles evenly and the cheese melts before the bread burns. Sizing the raw patty deliberately larger than the bread accounts directly for the shrinkage that happens as the beef renders fat and contracts during searing — a patty cut exactly to the bread's size at the start ends up visibly undersized by the time it's cooked.

Heat a cast-iron skillet or griddle over medium-high heat until very hot. Add a tiny amount of neutral oil only if needed. Sear the patties for 2–3 minutes on the first side until deeply browned. Flip and cook another 1–2 minutes until cooked through but still juicy. Transfer to a plate. Do not leave the patties sitting in their juices too long, or the bread will get soggy later. A hot pan and a thin patty together produce real browning quickly, which is what this sandwich needs given how little time the patty actually has before it's assembled into bread that also needs to stay dry.

Spread patty melt sauce on the inside of all 8 slices of brioche toast bread. Layer Swiss cheese on 4 slices, then add one beef patty, a generous amount of caramelized onions, and more Swiss cheese. Close with the remaining bread slices, sauce-side down. Cheese on both sides helps glue the sandwich together and gives a stronger cheese pull. Placing cheese directly against both the bottom and top of the filling means the melting cheese binds the whole stack together from both directions once griddled, rather than relying on the sauce alone to hold everything in place.

Spread softened butter on the outside of each sandwich. Heat a clean skillet or griddle over medium-low to medium heat. Cook the sandwiches for 3–5 minutes on the first side, pressing gently with a spatula, until the bread is deeply golden and crisp. Flip carefully and cook another 3–5 minutes, until the second side is golden and the Swiss cheese is fully melted. Do not use high heat. Brioche browns fast because of its sugar and butter. If the outside gets dark before the cheese melts, lower the heat and cover the pan for 1–2 minutes. Brioche's own sugar and butter content means it caramelises and browns considerably faster than a leaner bread would at the same heat — the moderate temperature specified here is calibrated specifically around that faster browning, giving the cheese and onions enough time to fully heat through without the bread darkening past golden.

Transfer the patty melts to a cutting board and rest for 1–2 minutes. Cut each sandwich in half. The short rest keeps the molten cheese and onions from flooding out immediately while still leaving a visible pull. Cheese and onions straight off the griddle are at their most fluid; the brief rest lets everything firm up just enough that the first cut doesn't send filling spilling out, while it's still hot and the cheese still pulls cleanly.

Serve immediately while the bread is crisp, the cheese is melted, the onions are jammy, and the beef is still hot. A patty melt gets worse fast once steam starts softening the bread. The hot filling continues releasing steam even after leaving the pan, and that steam works directly against the crisp crust the griddling stage just built, so serving without delay preserves it.