Saute onions in olive oil over medium-low heat. When onions are tender, transfer to bowl of electric mixer fitted with dough hook.

Add 1 cup of warm water to onion. Blend in yeast, sugar and salt. Crush 2 teaspoons fennel seeds using a mortar and pestle; add to onion mixture. Gradually mix in flour until you have a dough of medium-soft consistency. Turn out dough onto floured surface and knead until smooth and elastic, about 4 minutes. Cover dough and let rest for 20 minutes.

Grease a heavy baking sheet. Divide dough into 4 equal pieces and roll each piece into a ball. Flatten balls of dough to 3/4" thickness and place on greased baking sheet. Rub surface of each bread with remaining butter or oil and lightly sprinkle with remaining whole fennel seeds, pressing slightly to make seeds stick. Cover and let rise for 30 minutes.

Preheat oven to 425F. Bake breads until golden brown, about 25 minutes.