Separate cauliflower into individual florets–try to keep them uniform in size, approximately 1-2 inches across at the head.
Place florets in a large pot, cover with water and add 1 tsp kosher salt. Bring to boil, cover and cook 5-10 minutes. I cooked mine for about 8 minutes till they were crisp/tender. Remember they will have additional cooking in the oven.
Spray an 11 x 7-inch casserole with non-stick spray. Place cauliflower in prepared baking dish, making an even layer across the bottom. Set aside.
Preheat oven to 350 degrees F.
In a large saucepan over medium heat, melt 3 tbs butter. Add the peppers, onion and mushrooms to pan and saute until tender.
Add flour to pan with vegetables and blend well. Add the milk gradually, stirring constantly until well blended and there are no lumps. Add salt and pepper. Cook, stirring constantly, over medium heat until sauce begins to thicken. Add 3/4 cup of the cheese to the sauce. Stir until cheese melts.
Pour cheese/vegetable sauce over cauliflower, distributing the sauce evenly over the cauliflower. Cover with aluminum foil and bake for 20 minutes.
Melt the remaining 1 tbsp of butter.
Remove the casserole from the oven, uncover and distribute croutons evenly over the top. Drizzle with melted butter. Sprinkle with remaining 1/4 cup of the cheese then sprinkle the Parmesan cheese on top.
Return to oven and bake for an additional 10-15 minutes, or until cheese is melted and croutons are golden brown.
Serves 6-8 as a SIDE or 4-6 as a MAIN.
**Frozen cauliflower of equal measure may be substituted for fresh. Cook until crisp/tender, about 3 minutes**