In a small bowl, sprinkle yeast over warm water. Stir to combine and let sit until frothy, about 10 minutes.
In the bowl of large food processor fitted with a dough blade or a large bowl, combine flour, sugar, and salt. Mix in the butter and frothy yeast with water. If using the food processor, process until dough comes together.
On a lightly floured surface, knead dough until smooth and elastic. Place in oiled large bowl, turning once to coat. Cover and let rise until doubled, 45 minutes to an hour.
Place rack on lower third of oven and preheat to 450 degrees F. Lightly grease a large baking sheet with oil.
On a lightly oiled work surface, divide risen dough into 2 equal pieces. Shape each into an oval loaf and cover with towel. Let rise for about 10 minutes until puffy.
In a large pot, bring 10 cups of water to a boil, then add baking soda. Gently place one loaf in boiling water for 30 seconds, turning halfway. Remove with a large slotted spoon to prepared baking sheet. Repeat with other loaf. Brush each with egg wash and sprinkle with coarse salt. Cut a shallow "X" shape on top of the loaves or 3-4 parallel slashes across the top.
Bake in preheated oven until deep brown and hollow sounding in center, 20-25 minutes. Let cool on wire rack about 15 minutes before slicing.
Best served day of baking.