In a large bowl, combine both kinds of cheese, and cornstarch - set aside.
In a large saucepan, melt butter on medium heat. Stir in evaporated milk, garlic powder, and paprika.
Add the cheese mixture and stir until the cheese has melted (about 2-3 minutes) and the mixture becomes a thick, smooth sauce.
Remove from heat, add the broccoli & cauliflower into the sauce and toss to coat. Add to an 8-inch oven safe dish.
To make the topping:
Top with the grated Parmesan cheese
Bake 1 hour, or until golden and cooked through.
Notes
Always use a good quality cheddar to make a cheese sauce, it really makes a difference to the taste!
When steaming the cauliflower, steam until slightly tender, but not soft. It will finish cooking in the oven.
Have all your ingredients ready before you start the sauce. Things move fast when making the sauce. If you have to stop to grate cheese, it could end in disaster.
For an extra kick of flavor, use mature cheddar.
Add a breadcrumb topping to give the dish extra texture.
You can use or frozen broccoli and cauliflower that's thawed and drained well in place of steamed fresh.