Fill a large pot with about an inch of water and bring to a simmer. Fit a steamer basket over the pot.
Add the broccoli florets to the steamer, cover, and steam for exactly 5 minutes, until bright green and crisp-tender.
While the broccoli steams, melt the butter in a large skillet over medium-low heat. Once foaming, add the garlic and cook 30 to 45 seconds until fragrant, then remove from heat.
Remove the steamer basket and let the broccoli drain for 30 seconds, or pat dry with a paper towel.
Add the drained broccoli to the skillet with the garlic butter, off the heat, and toss to coat.
Add the parmesan, lemon juice, salt, pepper, and red pepper flakes. Toss again until the cheese melts into the sauce. Serve immediately.