The addition of Dijon mustard and Worcestershire sauce makes the cheese sing in this soup. The addition of spinach and roasted vegetable croutons adds nutrition without cutting the decadent creaminess of this soup. Perfect for a cold night supper!
If using fresh spinach, remove stems and wash thoroughly. Wilt, covered, in the microwave or over medium heat. If using frozen spinach, thaw. Place wilted or thawed spinach in a colander and press with a large spoon to remove as much water as possible. Set aside.
In a medium soup pot, melt the butter over low heat. Add the flour, salt and pepper to the melted butter and whisk to mix thoroughly, Cook, stirring periodically, for 3 minutes. Add the mustard and Worcestershire sauce and whisk to mix.
Optionally, heat the milk until hot but not boiling. Pour the milk, a little at a time, into the flour and butter mixture, whisking constantly. Allow to come just to a simmer, stirring frequently, to develop a cream sauce of medium thickness.
Add two cups water to the sauce and stir until mixed. Allow to come back to a slow simmer. Slowly add the grated cheese, stirring constantly, and allowing each addition of cheese to melt before adding more.
Mince spinach until very fine. Alternatively, pulse the spinach briefly in a food processor, adding a bit of the cheese mixture if necessary. Stir the spinach mixture into the cheese mixture.
To avoid scorching, do not boil the soup. Serve hot with the vegetable croutons.
TO MAKE THE CROUTONS: Preheat the oven to 375F. Toss the vegetable pieces in the pesto and olive oil. Toss the now coated vegetables in the crumbs. Roast the vegetables on a rack until the hardest of them are fork-tender. Top bowls of soup with the croutons.