Preheat oven to 375°F. Coat a baking dish with cooking spray and place them on a baking sheet.

Heat water in a pan and cook broccoli florets for 2 minutes, drain and keep aside.

Melt butter in a pan over slow-heat. Whisk in flour and cook for 1 minute.

Add milk, mustard powder and salt and cook keep whisking until thickened .

Remove from heat and add in cheese and 3 egg yolks until well combined. Keep it aside.

In a medium bowl take 5 egg whites and cream of tartar and beat with an electric mixer on high speed until soft peaks form.

Gently fold half of the whipped whites into the milk mixture.

Gently fold in the remaining egg whites and the reserved broccoli mix.

Transfer this mixture into prepared baking dish. Bake until puffed, firm to touch and browned. It will be approximately 15-20 minutes