In a small skillet toast the fennel seed over moderate heat until very fragrant, about 2 to 3 minutes. In a large saucepan melt the butter and cook the leek until very soft, about 5 minutes. Stir in stock and potatoes and bring to boil. Simmer the soup until the potatoes are tender, about 10-15 minutes. Stir in the toasted fennel, kielbasa, cream and salt and pepper, to taste. Before serving stir in the arugula. Garnish with red bell pepper.