Combine first 9 ingredients in heavy soup pot or Dutch oven. Bring to boil over high heat. Cover pot; reduce heat to low and simmer until peas are tender, stirring occasionally, about 1 hour.
Transfer sausage to large plate; cut into bite-size pieces. Reserve 1/4 of sausage pieces; return remaining sausage to soup. Add 1 cup Gouda to soup and stir until melted. Thin soup to desired consistency with water if necessary. Season with salt and pepper. Keep warm
Preheat broiler. Mix remaining 1 1/2 cups Gouda with reserved sausage pieces. Spread generous amount of mustard on bread slices. Top with Gouda-sausage mixture. Broil until cheese melts and begins to brown. Serve soup, passing toasts separately.
