Roast 5 tablespoons of flour in a casserole dish over high heat, stirring constantly, until hazelnut brown. Remove from heat and let cool for 1 minute.
Add 50 g butter or margarine and stir to combine.
Return the casserole to medium heat, add the onion strips, and cook for 2 minutes.
Pour in 1 liter of meat stock, increase heat to medium-high, and bring to a boil while stirring.
Reduce heat to medium-low and simmer for 1 hour, stirring occasionally.
While the soup simmers, grate 100 g of cheese.
Divide the soup into preheated plates and sprinkle the grated cheese on top, or serve the cheese separately.