In Dutch oven combine cauliflower and a small amount of lightly salted water. Bring to a boil; reduce heat. Simmer, covered, for 8 to 10 minutes or until tender. Drain cauliflower; set aside in medium bowl. In the same Dutch oven melt butter. Stir in flour, celery seed and pepper. Add half and half and broth all at once. Cook and stir until slightly thickened and bubbly. Cook and stir for 1 minute more. Add cooked cauliflower. Using a potato masher, mash some of the cauliflower to slightly thicken the soup. Add cream cheese and velveeta. Cook over very low heat for 5 minutes or until cheeses are melted, stirring occasionally. Season to taste with salt if desired.