Very slightly adapted from Faith Heller Willinger's Adventures of an Italian Food Lover. Serves 6-8.
3/4 lb head of cauliflower, broken into bite-size florets
3/4 lb broccoli, cut into bite-size florets, stems peeled if necessary
3 Tbsp coarse sea salt
1 cup non-fat or 1% milk
3 Tbsp extra virgin olive oil
1/4 cup all-purpose flour
3/4 cup grated Parmigiano-Reggiano cheese
Freshly ground black pepper, to taste
In a stock pot, bring 6 quarts of water to a rolling boil. Add 3 Tbsp coarse sea salt, and the cauliflower, and cook, uncovered, 10-12 minutes, until tender. Remove the cauliflower from the pot, place in a large bowl of ice water to stop the cooking, then drain and place in a large mixing bowl.
Add the broccoli to the same pot and cook, uncovered, for 10-12 minutes, until just tender. Remove broccoli from the pot, place in the ice water, then drain and combine with cauliflower in a large bowl. Reserve 1 cup of the vegetable cooking water.
Preheat the oven to 375°F.
Heat the milk and vegetable-cooking water in a small pot. In another small pot, add the olive oil and flour, and stir over low heat for a few minutes (to make a roux), but don't let the mixture color. Add the hot milk and stir energetically with a whisk until smooth and creamy. Remove from heat, stir in the cheese, and season with salt and pepper.
Combine the cheese sauce with the broccoli and cauliflower, and transfer to a baking dish (I used a 9x13 ceramic baker), smoothing to a 2-inch layer. Bake 20-25 minutes, until bubbling. Cool for 5 minutes before serving.