Preheat the oven to 190°C / 375°F / Gas mark 5.
Grease 4 ramekins or a soufflé dish with a little butter and place them on a baking tray.
Use a microwave safe bowl and cook the broccoli covered until tender yet crisp, about 2 minutes. Set aside.
Melt the butter and oil in a large saucepan over medium-high heat.
Whisk in flour and cook.
Reduce the heat then add the milk, mustard, rosemary and salt and cook, whisking constantly, until thickened.
Remove from heat and immediately whisk in the goat cheese and 3 egg yolks until well combined.
Transfer to a large bowl.
Beat the 5 egg whites in a medium bowl by hand or with an electric mixer on high speed until soft peaks form.
Add the cream of tartar and continue beating until stiff peaks form.
Using a rubber spatula and gently fold half of the whipped whites into the milk mixture.
Gently fold in the remaining egg whites and the reserved broccoli just until no white streaks remain.
Transfer to the prepared ramekins or soufflé dish.
Bake for 20 minutes in ramekins or 30 minutes in a soufflé dish.