Nick Kindelsperger of The Paupered Chef (and Serious Eats contributor) just made sliders at home for the first time, and although they look enticing in the photograph, he says the burgers came out slightly tough and dry. His method:
Our method was to create a "smash burger" with our usual technique, which creates a mouthwatering crust on one side, then to flip them and finish over onions. But by the time the onions were truly soft and fragrant, the burgers were overdone. And that caramelized exterior on the patty actually didn't really suit the slider, since the slider is all about softness. When I think of sliders I think of something that's nearly insubstantial. You need to eat a whole mess of them to fill up.
His readers have contributed some tips, such as slicing the onions super thin with a mandoline, letting the onions cook for a bit before adding the meat, or topping the patties with onions first, then flipping the patty and onions over. If you have slider-making tips, let Nick know!