Place a large saucepan or Dutch oven over medium heat and add about half of the olive oil.
Once hot, add the chopped onion and cook, stirring occasionally, for about 10 minutes until it softens and just begins to brown.
Stir in the minced garlic and red pepper flakes and cook for one more minute until fragrant.
Add the cauliflower florets, a generous pinch of salt, and black pepper.
Continue to cook, stirring frequently, for 3 to 5 minutes until the cauliflower is well-coated and glistens.
Pour in the vegetable stock and stir to combine. Increase the heat to high and bring the soup to a boil.
Reduce the heat to maintain a gentle simmer.
Cook for 10 to 15 minutes, or until the cauliflower is tender but not mushy.
Taste the soup and adjust the seasoning as needed.
Stir in the remaining olive oil just before serving.
To serve, place a large crouton in the bottom of each bowl and ladle the hot soup over it.
