See links to Sourdough Starter. Stir sourdough starter before measuring. Measure out 1-1/2 cups starter and bring to room temperature.
Combine 2 cups flour and salt in a large mixing bowl. Gradually add water, olive oil, and starter to flour mixture. Beat 2 minutes at medium speed of mixer, scraping bowl occasionally. Beat 2 minutes at high speed. Stir in enough remaining flour to make a soft dough. Knead on lightly floured surface until smooth and elastic, about 8 to 10 minutes. Place in lightly greased bowl, turning to grease top. Cover; let rise in warm, draft-free place until doubled in size, about 1 hour.
Punch dough down; remove to floured surface. Divide dough in half; roll each to 13x 6-inch rectangle. Roll up tightly from long end as for jelly roll. Pinch seams and ends to seal. Taper ends by rolling back and forth. Brush all surfaces with water. Combine all seeds together; roll loaves in seed mixture, coating completely. Place; seam sides down, on lightly oiled baguette pan or large baking sheet. Cover; let rise in warm, draft-free place until doubled in size, about 45 minutes.
With sharp knife, cut 3 slashes (1/4-inch deep) lengthwise down center, cutting at a slight diagonal. Bake in preheated 425°F oven for 25 to 30 minutes or until done. Remove from pan; cool on wire rack. Serve with additional olive oil for dipping.