Turn the stove on over medium heat and place a pot with 1 tbsp oil.
Add the chopped onion and cook until softened and translucent, about 4β5 minutes.
Stir in the carrot and celery and cook for 1β2 minutes.
Add the thyme and black pepper, stirring to coat the vegetables.
Add the cauliflower, potato, and tomato. Cook for 1 minute while stirring.
Pour in the chicken stock or vegetable broth. Add a little water if needed to fully cover the vegetables.
Bring to a gentle boil, then reduce the heat, cover, and simmer for 25β30 minutes, or until the vegetables are tender.
While the soup cooks, prepare the roux. Heat the remaining tablespoon of oil in a small saucepan over medium heat.
Add the flour and stir continuously until a smooth, lightly golden roux forms.
Slowly stir the roux into the soup and cook for an additional 10 minutes, stirring occasionally.
For a creamier texture, partially or fully blend the soup using an immersion blender.
Taste and adjust the seasoning with salt and pepper as needed.
Serve hot with fresh bread, Serbian pogacha, or a simple salad.