To blanch the vegetables: Bring a large pot of water to a vigorous boil. Add the broccoli and cauliflower. Cook for 2 minutes (for the broccoli) and 3 minutes (for the cauliflower).
Remove from the pot with a slotted spoon and immediately put them in a bowl of ice water. When cool drain completely and place in a gratin dish that has been coated with olive oil cooking spray.
In a medium saucepan melt 2 tablespoons of the butter over medium heat. Add the onions and cook until they are golden brown, stirring occasionally.
Add the flour and cook and stir for several minutes. Slowly pour in the milk stirring constantly to prevent lumps.
Slowly bring the mixture to a boil. Continue cooking and stirring until the sauce has thickened. Stir in the mustard.
Remove the pan from the heat and stir in the cheddar and blue cheese. Once the cheeses have melted, pour the sauce over the vegetables. Stir gently to coat everything evenly.
Melt the remaining butter in a small pan. Add the Panko crumbs and smoked paprika and toss until the crumbs are coated.
Sprinkle the crumbs over the casserole. Spray the top of the casserole liberally with olive oil cooking spray. Bake in a 375 degree oven until the vegetables are tender, the top is golden, and the sauce is bubbly. This should take 30 to 40 minutes.
Remove from the oven and allow to cool for 10 - 15 minutes before serving.