1. In a saucepan, over medium heat, melt butter, then add onions. Stir for 5 minutes, then add flour. Stir until well mixed.
2. Add chicken broth and stir continuously until homogeneous. Add milk. Mix well until the mixture starts to boil. Take out of heat.
3. Mash the mixture gradually. First, add one cup of the mixture in the blender and mash well over medium speed. Add the remaining quantity gradually until it becomes smooth. Return mashed mixture to the saucepan, over medium heat, and leave until boiling, then take out of heat.
4. Add shredded cheese to the mixture and stir manually until the cheese melts. You can put the mixture over heat for one minute until the cheese melts completely.
5. Pour the cheese soup in a serving dish, garnish with shredded cheese, thyme, croutons. Serve hot with crunchy toasts. .