Heat vegetable oil and butter in a heavy skillet over medium heat until butter has melted. Cook and stir onions in butter and oil until lightly golden brown, about 10 minutes. Push onions aside with a spatula.
Rub steaks with cut sides of garlic clove.
Place steaks in the skillet, leaving the onions to the side, and cook over medium-high heat until meat is browned but still lightly pink inside, 2 to 4 minutes per side.
Remove the skillet from heat. Slowly pour Irish whiskey into the hot skillet (be careful, whiskey fumes are flammable). Mix browned onions into whiskey and bring to a simmer over medium-low heat.
Sprinkle steaks with salt, black pepper, and parsley; turn steaks over in whiskey pan sauce to coat both sides, and serve drizzled with sauce.
