Philly Cheesesteak Sliders

Name: Philly Cheesesteak Sliders
Ingredients: 1.5 lb. boneless ribeye steak (about 2 [3/4-thick] steaks); 2 tbsp. unsalted butter, melted; 1 tbsp. chopped fresh parsley leaves; 1 clove garlic, finely chopped; 0.75 tsp. kosher salt, divided, plus more; 0.25 tsp. freshly ground black pepper, plus more; 3 tbsp. vegetable oil, divided; 1 medium yellow onion, sliced into half-moons (about 1 1/2 c.); 1 large green bell peppers, seeds and ribs removed, thinly sliced; 12 mini Hawaiian rolls; 0.25 c. mayonnaise; 8 slices American or provolone cheese