Preheat oven to 425˚F with rack at center.
Bring a large pot of water to a boil, 5-6 quarts.
Place 1 head of cauliflower florets, 2-3 cups, in a bowl and toss with EVOO and za’atar, salt and pepper. Scatter on large, parchment-lined baking sheet. Roast until tender and browned at edges, 25 minutes.
Salt boiling water and cook remaining head of florets, 10-12 minutes, until fork-tender and remove the cauliflower with a spider to transfer to a high-power blender or food processor.
Meanwhile, heat milk, butter and crushed garlic, seasoning the milk with salt, pepper and nutmeg. Add hot milk to blender or processor.
Add cheese to cauliflower and hot milk, then puree sauce until very smooth. Taste to adjust seasonings.
Cook fettuccine 1 minute less than package directions in the boiling, salted water, reserving ½ cup starchy water before draining.
Toss pasta with sauce and, if necessary, a splash of pasta cooking water.
Serve pasta in large bowl or individual shallow bowls topped with za’atar-roasted florets.