Melt the butter in a Dutch oven or large pot over medium heat. Add the onion and fennel and cook, stirring, just until the onion is translucent but not colored, about 5 minutes. Add the rice, season with salt and pepper and cook, stirring for 5 minutes longer. Add 1 cup of the hot stock. Cook, stirring often, until the rice absorbs most of the liquid. Add another cup of the hot stock and cook and stir as before. Continue cooking, stirring frequently and adding the stock 1 cup at a time, until 6 cups of stock have been added. At this point, taste the rice. Usually, the rice will need another 1/2- to 1-cup of stock and a few more minutes to cook. Stir in the fennel-parsley purée, the cheeses, fennel seeds and pastis. Taste and season with salt and pepper. Refrigerate until cold.