To make the Smoky Onion Mayo, add mayonnaise, chives, Smoky Onion Salt, vinegar, and hot sauce to a medium bowl and stir to combine. Taste and adjust seasoning and set aside.

To make the Bacon-Onion Jam, heat a Carbon Steel Griddle on the grill or stove top over medium. Add bacon and cook, flipping occasionally, until fat has rendered and bacon is as crispy as you like it, 3–4 minutes per side. Transfer bacon to a paper-towel lined plate and allow to cool slightly, then chop. Remove all but about 1 Tbsp. bacon fat from Griddle, then add onions. Season with Smoky Onion Salt and pepper. Cook until softened and caramelized, 10–15 minutes, adjusting heat if necessary. Add chopped bacon back to Griddle and fold in maple syrup. Cook until flavors meld, about 2 minutes. Transfer to a medium bowl and stir in vinegar. Taste and adjust seasoning.

To make the patty melts, divide ground beef into 4 equal, lightly packed, ball-shaped portions (they are not patties—yet). Slather mayo on what will be the tops and bottoms (but not insides) of your soon-to-be sandwiches.

Heat Griddle over medium-high and lightly drizzle the surface with oil. Use a spatula, heatsafe brush, or paper towel to coat evenly.

Place beef portions on griddle and smash flat with a parchment-lined or oiled Grill Press to form thin patties, between ¼” and ½”-thick. Try to match the size and shape of your bread. It’s ok if your patties are not perfectly round, the uneven edges will become browned and crispy. Season generously with Smoky Onion Salt and pepper and cook until edges are brown and juices start to accumulate on top, about 2 minutes.

Flip patties with a large heatproof Spatula (do not smash again), season lightly with Smoky Onion Salt and pepper, and allow to cook about 1 minute more. Place a slice of cheese on top of each patty. Cover and cook until cheese starts to melt and burgers are about medium, 1–2 minutes more. Transfer to Sheet Pan or Platter.

Wipe Griddle clean, if necessary, and toast bread, mayo-side-down. Working quickly, build patty melts by spreading Smoky Onion Mayo on the bottom slices of bread, then topping each with 2 patties (they can be stacked slightly askew to fill the sandwich, if necessary), Bacon-Onion Jam, more Smoky Onion Mayo, and finally the tops, toasted side up. Press tops gently with Grill Press. Transfer to plates and serve.