Place ground beef in a bowl. Add salt, black pepper, garlic powder, onion powder, and Worcestershire sauce. Mix gently with your hands or a fork until just combined — avoid overworking to keep patties tender.

Form patties

Divide the mixture into 12 equal portions (about 1.3 oz / 37 g each). Roll each portion into a loose ball, then flatten into thin patties about 2.5–3 inches in diameter and slightly larger than the slider bun so they fit the bun when cooked (they will shrink). Make a slight dimple in the center of each patty with your thumb to reduce doming.

Preheat pan or griddle

Heat a large skillet or electric griddle over medium-high heat. Add the vegetable oil and spread to coat the surface. The pan is ready when a drop of water sizzles on contact.

Cook patties

Working in batches if necessary, place patties on the hot surface without crowding. Cook for 2–3 minutes on the first side until well-browned. Flip and cook another 1.5–2 minutes for medium, a bit longer if you prefer more done. During the last 30–45 seconds of cooking, place a halved American cheese slice over each patty to melt, and cover loosely with a lid or tent with foil to speed melting.

Buns & Assembly

Prepare buns

Preheat the oven to 350°F (175°C). Split the slider buns if not pre-split. Brush the cut sides of the tops and bottoms lightly with melted butter. If using sesame seeds, sprinkle a pinch on the top halves.

Warm buns

Arrange the bottom halves on a baking sheet (or keep top halves nearby). Place in the oven for 3–5 minutes until the buns are lightly toasted and the butter soaks in. Remove from oven.

Assemble sliders

Spread mustard and/or mayonnaise on the bottom bun (about 1/4–1/2 tsp each as desired). Place a cheeseburger patty on each bottom bun. Top with a pickle slice, a small amount of ketchup if desired (about 1/4 tsp), and a thin slice of tomato and red onion if using. Cover with the bun top.

Optional finish

If making a platter, place assembled sliders back in the oven for 2–3 minutes to meld flavors and fully warm through. Sprinkle chopped parsley over the tray before serving.

Serving

Arrange sliders on a platter and serve immediately. Provide extra ketchup, mustard, and pickles on the side. Leftovers can be refrigerated in an airtight container for up to 2 days; reheat in a 325°F (160°C) oven for 8–10 minutes or in a skillet until warmed through.