Cream of Kohlrabi Soup

2 Tbsp butter
1 lb leeks or onions, thinly sliced
2 kohlrabi, trimmed, peeled and cut into 1/4 inch dice
1 large potato, peeled and cut into 1/4 inch dice
1 quart chicken stock
1 Tbsp chopped fresh thyme (or 1 tsp dried)
1/2 cup heavy cream
salt and freshly ground pepper

Melt the butter in a medium size heavy kettle over medium heat. Add the leeks (or onions) and cook, stirring, until translucent and tender, 10-15 minutes. Stir in the kohlrabi and potato and cook, stirring 3to 4 minutes longer. Stir in the stock and thyme. Simmer over low heat until the kohlrabi and potato are very tender, 20-25 minutes.Remove about half of the vegetables and purée them in a food processor of blender, adding a little of the stock if necessary. Return the purée to the kettle and stir in the cream. Season with salt and pepper and simmer over low heat for 10 minutes longer. Serve very hot.