Image credit to: Katie Bornn
Chop the cauliflower and celeriac into roughly the same size chunks. Toss in olive oil, salt and pepper and arrange them in a single layer on a large baking sheet. Roast at 400F oven until lightly golden brown, about 20-30 minutes.
Heat the oil in a large sauce pan over medium heat, add the onion and saute until tender, about 5-7 minutes.
Add the garlic and thyme and saute until fragrant, about a minute.
Add the broth, deglaze the pan, add the cauliflower & celeriac, bring to a boil, reduce the heat and simmer, covered, for 20 minutes.
Puree the soup until it reaches your desired consistency with an immersion blender.
Mix in the cheese, let it melt without bringing it to boil again.
Mix in the milk, season with salt and pepper and remove from heat.