Preheat the oven to 350°F (175°C). Coat a 9x13-inch baking dish with nonstick cooking spray.

Fill a large pot about three-quarters full with water, add the salt, and bring to a boil over high heat. Add the cauliflower florets and cook for 8 to 10 minutes, until just fork tender. Drain very well in a colander and let the florets steam dry for a few minutes, then spread them in the baking dish.

Scatter the ham over the cauliflower, tucking the cubes in between the florets.

Meanwhile, melt the butter in a large skillet over medium-low heat. Add the heavy cream and bring it to a gentle boil, stirring often, then lower the heat and simmer for 10 minutes. Stir in 2 ½ cups of the cheddar until melted. Keep simmering, stirring often, for another 3 to 4 minutes, until the sauce coats the back of a spoon and starts to cling to the sides of the skillet.

Pour the cheese sauce evenly over the cauliflower and ham, then sprinkle the remaining ½ cup of cheddar over the top.

Bake, uncovered, for 40 to 45 minutes, until the casserole is bubbly and the top is golden in spots. Let it rest for 10 minutes so the sauce can thicken, then sprinkle with sliced green onions and serve.