Put the bread slices onto a sheet pan and toast in the oven until golden; set aside until cool enough to handle.
Pour the milk and stock into a large saucepan; add a little salt, and bring to a simmer over medium heat.
Add the cauliflower to the milk mixture; add the onion, thyme sprigs, butter, and bay leaf; bring to a simmer, cover and cook for about 15 minutes, until the cauliflower is tender.
Remove thyme and bay leaf and puree the soup in a blender.
Pour the soup back into a saucepan and check for seasoning.
In the meantime, grate the toasted brioche slices on a box grater to make the crumbs; drizzle with the butter and toss.
Spread the mixture on a sheet pan and toast in the oven until lightly browned and crispy; remove from oven, scrape into a bowl and stir in parsley.
To serve, pour the soup into bowls and sprinkle with the brioche crumbs and crispy bacon.