Steam over boiling water for 5–6 minutes until bright green and just tender. Do not overcook — the florets should hold their shape and have a slight bite.
While broccoli steams, make the cheese sauce: melt butter in a small saucepan over medium heat.
Whisk in flour and cook 90 seconds, stirring constantly, to remove the raw taste.
Slowly pour in milk while whisking constantly. Keep whisking until the sauce is smooth and comes to a gentle simmer.
Cook 3 minutes until thickened enough to coat the back of a spoon.
Remove from heat. Add shredded cheese and stir until fully melted. Season with salt.
Arrange steamed broccoli on a serving dish and pour cheese sauce generously over the top. Serve immediately.