Melt 2 tablespoon of the butter or margarine in a saucepan. Add the carrots, potatoes and onion and fry (sauté) over a fairly high heat for minutes, stirring.
Stir in the cumin and cook for 1 minute, Add the stock, bay leaf, cayenne and a little salt and pepper. Bring to the boil, reduce the heat until gently bubbling around the edges and cook for 20 minutes. Discard the bay leaf.
Puree the soup in a blender or food processor and return to the saucepan. Stir in the milk, taste and re-season if necessary.
Meanwhile, heat the remaining butter or margarine and the oil in a frying pan (skillet), and fry the bread cubes with the garlic over a fairly high heat, stirring and turning until golden. Drain on kitchen paper (paper towels) and discard the garlic.
Ladle the soup into bowls and sprinkle with croutons.