The original recipe for this came from a Bon Appetite, maybe thirteen years ago? I have made it so often I no longer need to look it up, and, like stew, each time is a little different. If you internet search I am sure you can find the original and you could compare it to this. It's all good.
1 head cauliflower
2 medium heads broccoli
5 ounces baby spinach
1/2 onion, diced small
2 cloves garlic, minced
4 Tablespoons butter
1/4 cup flour
1 cup chicken or vegetable broth
1 cup half and half or milk
1 cup reserved vegetable cooking water
1/8 teaspoon grated nutmeg
1 double handful shredded Italian Five cheeses
1 cup Panko
1/2 cup grated Parmesan cheese
spray olive oil
salt and pepper
Preheat the oven to 375 degrees.
Remove the florets of cauliflower and broccoli from the stems, dividing the larger pieces into bite size pieces. Keep the two vegetables separate.
In a large pot (I use a dutch oven) bring 5 cups of heavily salted water to a rolling boil. Cook the cauliflower to crisp tender and remove from the pot with a spider. Place the vegetable in ice water to stop the cooking. Bring the water back to the boil and cook the broccoli identically and chill it as well.
Bring the water back to the boil. Put the spinach in the water and use the spider to dunk it until it wilts. Remove the spinach and turn off the heat under the water. By now the water will be tinged green. Reserve 2 cups of the liquid and pour the rest away. Put the dutch oven back on the stove and when the residual heat in the pot dries it, add the butter and turn the heat back on to medium low.
When the butter melts and begins to bubble, add the onion and stir until the onions turn translucent, 2 or 3 minutes. Add the garlic, flour, and nutmeg and stir as you cook the roux for three minutes. Add a large pinch of salt and and pepper. Add the chicken stock and stir as the mixture begins to boil. Add the dairy and continue to stir. Add one cup of the reserved cooking water and stir as the sauce comes to the boil. Add the Italian 5 cheeses and stir until the cheese has melted. If the sauce is tight, add more of the reserved cooking water. Taste test and adjust the salt if necessary. Turn off the heat.
Spray grease a 9 X 13 casserole. Add the cauliflower, broccoli, and spinach into the sauce and mix well. Turn the mixture out into the greased casserole. Mix the Panko and Parmesan together and thoroughly cover the top of the vegetables. Use spray olive oil to dampen the breadcrumb mixture.
At this step you can cover the casserole and hold it, refrigerated, for up to 48 hours.