Place the cauliflower florets in a large saucepan and cover with water.

Bring to a boil, and cook until tender, about 10 minutes.

Drain well.

In a medium saucepan, heat the olive oil over medium heat.

Add the garlic and scallion and cook for about 2 minutes, until fragrant.

Stir in the tahini, cumin, paprika, cayenne, salt, and flour.

Cook for 1 minute.

Whisk in the milk and cook until it begins to thicken.

Combine the cauliflower florets and sauce in a blender or use an immersion blender to purée until smooth.

Preheat the oven to 350°F (180°C, or gas mark 4).

Lightly coat a 9 × 13-inch baking dish with nondairy butter.

Cook the pasta according to package directions until al dente.

Reserve some cooking water to thin out the sauce, if needed.

Drain the pasta and return it to the pot.

Stir the cauliflower sauce into the pasta, adding reserved liquid if necessary.

Pour the pasta mixture into the baking dish.

In a food processor, blend walnuts, nutritional yeast, lemon zest, thyme, pepper, and salt into a coarse meal.

Combine with the bread crumbs in a bowl.

Use a fork to coat the mixture evenly with olive oil.

Sprinkle the bread crumb topping evenly over the pasta.

Bake for 15 minutes, until the topping is golden brown.

Let the dish stand for at least 15 minutes before serving.