Houston Restaurant's Canadian Cheese Soup

Melt the margarine in a Dutch oven. Add the carrots, onions and celery all at once and sauté until soft but not brown. Add flour and stir to combine. Cook until mixture begins to turn a light-brown color. Over medium-high heat, add the chicken broth a little at a time. Stir and continue to cook and stir with a whisk until a thick base if formed. Add half-and-half being careful not to let it boil. Add cheese, stirring until melted. Just before serving add parsley. Garnish with diced tomatoes and jalapeños, if desired.