- Cook pasta in a large pot of salted water according to package instructions. Drain and set aside.
- Combine milk, cream, garlic, and a pinch of salt in a medium saucepan over medium-low heat. Bring to a gentle simmer.
- Add cauliflower, cover, and cook until just tender, about 4 minutes. Remove half the cauliflower and set aside.
- Continue cooking remaining cauliflower, covered, until soft, about 8 additional minutes. Remove garlic from cream and discard.
- Working in batches, transfer cream-cauliflower mixture to a blender and blend until smooth.
- In a large bowl, toss pasta with cream-cauliflower mixture, reserved cauliflower, and thyme. Season with salt and pepper.
- Heat broiler. Divide pasta between 4 gratin dishes. Top each with cheese, thyme leaves, and sage. Broil until cheese is browned and bubbling, about 5 minutes.