While whisking, slowly pour in milk & add nutmeg. Whisk constantly until mixture reaches a boil, then remove from heat & stir in 1 1/4 cups cheddar cheese & the parmesan. Season with salt & pepper to taste.
Pour & spread 1/3 of the cheese sauce into a 9 x 9-inch baking dish. Top with drained broccoli/cauliflower combo then slowly & evenly pour remaining 2/3 of the cheese sauce over top.
Sprinkle over remaining 1/4 cup white cheddar then sprinkle evenly with panko crumbs. Spray panko with olive oil cooking spray to lightly coat.
Bake for 20 minutes, then broil for 1-2 minutes to help brown further as needed. Remove from oven, sprinkle with bacon & parsley & serve warm.