Bistec Encebollao

Ingredients

• 2 pounds beef sirloin steak, sliced thinly across the grain
• 1/2 cup olive oil
• 2 tablespoons minced garlic
• 1 pinch dried oregano
• 1 (.18 ounce) packet sazon seasoning
• 2 large white onions, sliced into rings
• 1/4 cup distilled white vinegar
• 1 cup beef stock
• 1 teaspoon salt