Place the broccoli florets in a large saucepan and add about 1 cup of water. Cover the pan and bring it to a simmer over high heat. Steam for 4-5 minutes until the broccoli is crisp-tender and bright green.

Carefully drain the steamed broccoli in a colander, ensuring to eliminate excess water. Transfer to a bowl and cover with a kitchen towel.

In the same saucepan, melt 2 tablespoons of butter over medium-low heat. Sprinkle in 2 tablespoons of flour, whisking for about 1 minute until golden.

Gradually whisk in 1 cup of milk, avoiding lumps, and let it come to a gentle simmer, cooking for 3 minutes until thickened.

Reduce heat to low and slowly whisk in 1 cup of shredded cheddar cheese until melted completely. Stir until smooth and creamy.

Stir in ¼ cup of grated Parmesan cheese, ½ teaspoon of kosher salt, ½ teaspoon of ground mustard, and a pinch of cayenne.

Pour the cheese sauce over the warm broccoli or toss together, and serve immediately.