Preheat oven to 375 degrees. Butter a 9″x13″ pan, and set it aside.
Melt butter in a pan. Stir in garlic until fragrant for about 30 seconds. Sprinkle flour over the top and whisk to coat. Cook flour mixture for about a minute while stirring.
Slowly stir in chicken stock to prevent lumps and then milk. Bring to a simmer and cook until thickened about 3 minutes. Remove from heat and stir in 2 1/2 cups Grated Cheese.
Place half of the cut Cauliflower or broccoli/cauliflower mix into buttered pan. Cover with half of the cheese sauce. Then repeat with remaining broccoli and cheese sauce. Sprinkle with the remaining 1 cup of grated cheese.
In a small bowl mix together panko bread crumbs with 2 tablespoons melted butter. Sprinkle panko crumbs, along with the Parmesan and Parsley, evenly over the top of the casserole.
Bake for 25-30 minutes, until veggies are tender and the top is lightly browned.
Allow to cool for 5-10 minutes before serving.