Pat the sirloin steak strips dry with paper towels and season with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat until shimmering.
Add the steak strips to the skillet in a single layer and cook for 2-3 minutes on each side until browned and cooked to desired doneness. Remove steak from skillet and set aside.
In the same skillet, add 2 tablespoons of butter and minced garlic. Sauté for 1 minute until fragrant, stirring constantly to prevent burning.
Add the broccoli florets to the skillet, season with salt, black pepper, and crushed red pepper flakes. Stir well to coat with garlic butter.
Cover the skillet with a lid and cook for 4-5 minutes, stirring halfway through, until broccoli is tender-crisp.
Return the cooked steak strips to the skillet and add the remaining 1 tablespoon of butter, lemon zest, and lemon juice.
Toss everything together gently and cook for an additional 1-2 minutes to combine flavors and heat through.
Remove from heat and sprinkle with chopped fresh parsley before serving.