Steam cauliflower until crisp-tender.
Preheat oven to 400 degrees and grease a casserole dish. I use one slightly smaller than a 9X13-inch pan.
In a large skillet melt butter over medium heat. Add green onions and garlic and cook 1 minute.
Add flour and cook, stirring constantly for 1 minute.
Gradually whisk in heavy cream and milk. Cook until bubbly and thickened.
Stir in salt, black pepper, dried sage, and red pepper.
Add cheddar cheese and remove from heat. Stir until melted. Check for seasoning and add more salt if desired.
Add cauliflower to cheese mixture and then pour entire mixture into prepared casserole dish.
In a small bowl, combine Panko crumbs, Parmesan cheese and melted butter. Scatter over top of cauliflower. Bake uncovered for 30 minutes.