Preheat a skillet over medium-high heat. Season the sirloin steak with salt and pepper on both sides. Add olive oil to the skillet and sear the steak for about 4-5 minutes on each side for medium-rare, or until desired doneness. Remove from the skillet and let it rest for 5 minutes before slicing.

In the same skillet, add minced garlic and sauté for about 30 seconds until fragrant. Add the chopped spinach and cook until wilted, about 2-3 minutes.

Pour in the heavy cream and bring to a simmer. Stir in the grated Parmesan cheese, lemon juice, and red pepper flakes (if using). Cook for an additional 2-3 minutes until the sauce thickens slightly.

Slice the rested steak and serve it topped with the creamy spinach sauce.