In a small bowl, mix melted butter with minced garlic, parsley, mustard, red pepper flakes, lemon juice, paprika, salt, and pepper. Set aside.
Pat the steak dry and season both sides with salt and pepper.
Heat olive oil in a cast iron skillet over medium-high heat. Sear steak for about 3–4 minutes on each side (medium-rare). Let rest before slicing thinly against the grain.
Toast slider buns cut side down in the same skillet until golden brown.
Assemble sliders by adding sliced steak and spooning cowboy butter on top of each bottom bun before adding optional toppings and placing the top bun.