Toss the cauliflower florets with 1 tbsp of olive oil, salt, and pepper.
Spread them out on a baking sheet in a single layer.
Roast in the preheated oven for 25-30 minutes, or until the cauliflower is tender and slightly browned.
Prepare the Mirepoix
While the cauliflower is roasting, heat the remaining 1 tbsp of olive oil in a large pot over medium heat.
Add the diced shallots, carrots, and celery to the pot.
Cook, stirring occasionally, until the vegetables are softened and the shallots are slightly caramelized, about 8-10 minutes.
Add the minced garlic, cumin, and coriander, and cook for another minute until fragrant.
Combine and Simmer
Once the cauliflower is roasted, add it to the pot with the mirepoix.
Pour in the vegetable stock and bring to a boil.
Reduce the heat and let it simmer for 15-20 minutes to allow the flavors to meld together.
Blend the Soup
Remove the pot from heat.
Use an immersion blender to blend the soup until smooth. Alternatively, you can transfer the soup to a blender in batches and blend until smooth. Be careful with hot liquids.
If the soup is too thick, add a bit more vegetable stock or water until you reach your desired consistency.
Season and Finish
Stir in the tsp of Vegemite (if using) until fully dissolved.
Taste and adjust seasoning with more salt and pepper if needed.
Serve
Ladle the soup into bowls.
Garnish with fresh parsley.
Serve hot, with crusty bread on the side if desired.