Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
Toss the cauliflower florets with half of the olive oil, sea salt, and half of the black pepper on the baking sheet.
Roast the cauliflower for 20-25 minutes, tossing halfway through, until the edges are golden and caramelized.
While the cauliflower is roasting, slice the beef top sirloin into thin, uniform strips.
Season the beef strips with the remaining black pepper and a pinch of salt.
Heat a large cast-iron skillet or non-stick pan over high heat with the remaining olive oil.
Once the pan is smoking slightly, add the beef strips in a single layer, working in batches if necessary to avoid crowding.
Sear the beef for 1-2 minutes per side until a deep brown crust forms but the center remains tender.
Add the minced garlic to the pan during the final 30 seconds of searing, tossing constantly to prevent burning.
Remove the pan from the heat and drizzle the beef with fresh lemon juice.
Serve the seared beef strips immediately alongside the roasted cauliflower.