Steam broccoli and cauliflower until just tender. Sprinkle with a little salt.
Heat oven to 375 degrees a grease a casserole dish.
In a large pan, melt butter over medium heat. Add flour and cook and stir for 1 minute. Gradually whisk in milk. Simmer for 2-3 minutes to thicken.
Remove pan from heat and stir in 2/3 cup Parmesan cheese, mushrooms, salt and pepper.
Add cooked fettuccine and broccoli and cauliflower to pan and stir to mix in. Transfer to prepared casserole dish and top with Panko crumbs and 2 tablespoons grated Parmesan cheese.
Place in oven for 25 to 30 minutes, until hot and bubbly.