Heat the oven to 425°F. Lightly oil a 2-quart baking dish and set it near the stove.

Fill a large pot with water, add a generous pinch of salt, and bring it to a boil over high heat.

Add the cauliflower florets to the boiling water in the pot.

Cook the cauliflower for 4 to 5 minutes, until it is just tender when pierced with a fork but not falling apart.

Drain the cauliflower in a colander and let it sit while you make the filling.

Set a large skillet over medium heat and add 1 tablespoon of the olive oil.

Add the chopped onion and 1/2 teaspoon of the kosher salt to the skillet. Cook for 6 to 7 minutes, stirring often, until the onion is soft and lightly golden at the edges.

Add the dried thyme and black pepper to the skillet. Stir for 30 seconds, just until the thyme smells fragrant.

Lower the heat to medium-low. Add the cream cheese pieces and the milk to the skillet.

Stir the cream cheese and milk in the skillet until the cream cheese melts and the sauce looks smooth, 2 to 3 minutes.

Add the drained beans and another 1/2 teaspoon of the kosher salt to the skillet. Cook for 2 minutes, stirring, until the beans are hot.

Add the spinach to the skillet in a few handfuls. Stir after each addition until the leaves wilt down into the sauce.