In a small bowl, whisk together the mayonnaise, minced chipotle in adobo (or paste), lime juice, minced garlic, smoked paprika, and 1/4 tsp salt until smooth. Taste and adjust heat or salt. Cover and refrigerate while you prepare the rest (at least 10 minutes for flavors to meld).

Caramelized Onions

Caramelize onions

Heat 1 tbsp vegetable oil (or 1/2 tbsp oil + 1/2 tbsp butter) in a skillet over medium-low heat. Add thinly sliced onion and a pinch (about 1/8 tsp) of salt. Cook slowly, stirring every 2–3 minutes, until onions turn deep golden brown and sweet, about 20–25 minutes. If they begin to stick, add a splash (1–2 tsp) of water and continue. Remove from pan and keep warm.

Burgers & Assembly

Portion and season beef

Divide the 1 lb ground beef into four loose 4-oz balls (makes four thin patties). Do not overwork the meat. Season the outside of each ball lightly with the 1 tsp salt and 1/2 tsp black pepper.

Preheat griddle or skillet

Heat a heavy cast-iron skillet or griddle over medium-high heat until very hot (2–3 minutes). Add 1 tbsp neutral oil or butter and spread to coat.

Smash the patties

Place two beef balls on the hot surface (do not crowd). Using a heavy spatula or a flat metal press and a piece of parchment or a spatula cover, firmly press each ball into a very thin patty (about 1/4-inch thick). Hold pressure for 10–15 seconds to get good edge sear. Sprinkle a light pinch of salt and pepper on the exposed tops immediately after smashing.

Cook patties

Cook the patties undisturbed for 2–3 minutes until the edges are deeply browned and crisp. Flip each patty, immediately place a slice of cheddar on top of each flipped patty, and cook another 45–60 seconds until cheese melts and the patty is cooked through (internal temp ~160°F / 71°C for medium-well). Work in batches for remaining patties, keeping finished patties on a warm plate.

Toast bread

While finishing the last patties, melt remaining 1 tbsp unsalted butter in a clean skillet over medium heat. Butter one side of each bread slice, then place buttered side down and toast until golden, 1–2 minutes. Butter the other side lightly and toast briefly if you prefer extra crispness.

Assemble patty melts

Spread about 1 tbsp chipotle mayo on the unbuttered side of each bottom slice of bread. Top two bottom slices with a cheese-topped patty each (for a single-patty sandwich use one patty), then divide caramelized onions evenly over the patties, add 4 pickle slices per sandwich, and top with remaining bread slice, mayo side inward. Press gently.

Final press (optional)

Return assembled sandwiches to the skillet buttered-side down and press with a spatula or skillet lid for 30–60 seconds to warm through and crisp the exterior. Flip and crisp the other side if needed, about 20–30 seconds.

Serve

Cut each patty melt in half on a bias and serve immediately with extra chipotle mayo on the side.