Slice the whole-wheat slider rolls horizontally, keeping the sheet intact. Place bottoms on a parchment-lined sheet pan.
Heat 1 tablespoon oil or butter in a large skillet over medium heat. Add diced onions, a pinch of salt, cook 5–7 minutes until soft and lightly golden. Set aside.
In a bowl, gently mix lean ground beef with 1 teaspoon kosher salt, 1/2 teaspoon pepper, 1/2 teaspoon garlic powder, and 1 teaspoon Worcestershire (if using). Don't overmix—tenderness matters.
Divide into 12 equal balls (about 1.5–1.7 oz each). Lightly press into thin rounds slightly larger than your rolls—they'll shrink.
Heat a large cast iron or heavy skillet over medium-high until hot. Lightly oil.
Add 4–6 patties at a time. Immediately press gently with a spatula to thin them. Cook 1.5–2 minutes until edges crisp.
Spoon a little cooked onion onto each patty, then flip. Cook 45–60 seconds more.
Top each with half a cheese slice. Add a teaspoon of water to the pan, cover for 20–30 seconds to melt. Remove to a plate. Repeat with remaining patties.
Pop roll bottoms in a 375°F oven for 3–4 minutes for a light toast. This keeps them from getting soggy.
Place cheesy patties on the roll bottoms, add pickles, and a swipe of mustard or special sauce. Cap with roll tops.
Return to the oven for 3–4 minutes to heat through. Slice into 12 sliders with a serrated knife. Serve immediately while the cheese is melty and the edges are crisp.