Preheat the oven to 400 degrees F (200 degrees C).

Place cauliflower and onions on a baking sheet. Drizzle with olive oil and sprinkle with cumin, coriander, salt, and pepper; mix to coat.

Place in the preheated oven and roast until cauliflower is browned and cooked, but not too soft, about 25 minutes.

While cauliflower is roasting, melt butter in a large saucepan over low heat. Add garlic and cook until fragrant, about 3 minutes. Stir in potato and turmeric. Add stock and increase heat to medium; bring to a simmer. Cover and cook until potato is soft, about 10 minutes.

Remove cauliflower and onions from the oven. Place 1 cup cauliflower in a bowl and set aside. Add remaining cauliflower and onions to the soup. Return soup to a simmer and cook until flavors have melded, about 5 minutes.

While soup is simmering, place almonds in a dry skillet over low heat. Cook, stirring frequently, until golden, 3 to 5 minutes.

Puree soup with an immersion blender until smooth. Adjust seasoning as needed. Ladle into bowls, add reserved cauliflower, and top with toasted almonds.