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 3-4 minutes without stirring to develop a sear.
Flip the steak strips and cook for an additional 2-3 minutes until browned but still slightly pink inside. Remove steak from the skillet and set aside.
In the same skillet, add the broccoli florets and sauté for 3-4 minutes until bright green and slightly tender.
Push the broccoli to one side of the skillet and add the butter and minced garlic to the empty side.
Cook the garlic in butter for about 1 minute until fragrant, stirring constantly to prevent burning.
Add the lemon zest, lemon juice, and crushed red pepper flakes to the skillet and stir to combine with the butter and garlic.
Return the cooked steak strips to the skillet and toss everything together to coat in the lemon garlic butter sauce.
Cook for an additional 1-2 minutes until the steak is heated through and the broccoli is tender-crisp.
Remove from heat and sprinkle with chopped fresh parsley before serving.