Preheat oven to 425 degrees and line a large baking sheet with parchment paper. 

Place the cauliflower florets, sliced onion, and peeled garlic in a single layer on the prepared baking sheet and toss with the avocado oil. Roast the vegetables for 30-35 minutes, until the cauliflower is becoming golden brown and softened. Set aside. 

When the vegetables are just about done roasting, heat the butter in a large pot or Dutch oven over medium heat. Add the shredded carrots and diced celery and saute until tender, about 2-3 minutes.

Add the roasted vegetables, vegetable broth, fresh thyme, salt and pepper to the pot. Bring to a simmer, then allow to simmer for 10 minutes, uncovered, stirring occasionally. 

Remove the pot from heat, and blend the soup with an immersion blender or transfer to a blender and blend until smooth. 

Return the soup back to the pot and stir in the cheese and heavy cream. Heat again over medium heat until warmed through. Garnish with more thyme leaves, and serve!