Trim the stems from the fennel and peel the bottoms. Chop the fennel fronds and reserve. Chop the bulbs and place them in a medium saucepan, along with the potato. Add the chicken stock. Heat to boiling; reduce the heat. Simmer, covered, until tender, about 15 minutes. Drain, reserving 1/4 cup of the cooking liquid.

Place the vegetables in the container of a food processor or blender. Add the reserved cooking liquid and process until smooth. Transfer to a large bowl. Stir in 1 tbsp of the chopped fennel fronds, the salt, and the fennel seeds.

Combine the yeast with the water and sugar in a small bowl. Let it stand (proof) 10 minutes, then stir it into the fennel purée. Add enough flour to make a fairly stiff dough, about 3 cups.

Transfer the dough to a lightly floured board and add the remaining 1 cup flour, kneading for about 10 minutes. Use the 1 teaspoon butter to butter a large bowl. Place the dough in the bowl, turning it to coat it with butter. Cover, and let rise until doubled in volume, about 1-1/2 hours.

Punch down the dough; knead briefly. Form it into a loaf and place it in a buttered 9-inch bread pan. Cover with a floured cloth; let stand 1 hour.

Preheat the oven to 375°F. With a sharp knife, cut a slash down the center of the risen dough. Brush the top well with the melted butter. Place a roasting pan containing 1 inch of water on the bottom of the oven. Place the bread on a rack in the top third of the oven. Bake until golden brown and hollow-sounding when tapped with your finger, about 45 minutes. Cool on a rack for 10 minutes. Gently loosen the bread from the pan and unmold. Cool completely on the rack.