For the Poolish:
In a very clean bowl, using clean utensils, mix flour, water, and yeast until well combined. Cover and let stand at room temperature 3 to 4 hours, or refrigerate overnight.
For the Lavender Water:
In a small pot combine lavender flowers and water and bring to a simmer. Remove from heat and leave to infuse until water has cooled to room temperature. Pass through a chinois.
For the Levain:
In a clean bowl, using clean hands, mix 1 tablespoon Poolish with flour and 220 grams Lavender Water, at 80°F. Cover bowl with a clean kitchen towel or cheesecloth and set in a warm place for 12 hours.
For the Baguettes:
Pour 500 grams water in a large bowl. Test Levain by placing a small bit in the water: if it floats it’s ready to use. If it sinks, discard half the mass of the Levain and mix 100 grams warm water and 100 grams all-purpose flour with the remainder. Rest until it passes the float test, about 2 hours.
When Levain is ready, scale 400 grams and add to water. Add 400 grams Poolish and stir together to bread up lumps. Add all-purpose and bread flours and mix with hands until flours are incorporated and dough is cohesive. Scrape down sides of bowl, cover with a towel, and rest 40 minutes.
Sprinkle salt evenly over surface of dough, then pour over remaining 50 grams water and squeeze mixture through the dough until well combined. Continue mixing as dough becomes stringy, then cohesive once again. Scrape down sides of bowl, cover with towel, and rest 30 minutes.
Lightly oil a work surface and scrape the dough out of the bowl. Gently flatten dough into a rough square; then fold the right third of the square into the center, followed by the left third, like folding a letter. Fold the bottom third of this rectangle up onto the center; then fold the top third down over it to create a square. Return dough to bowl, cover with towel, and rest 30 minutes. Complete two more such folds, with another 30 minute rest period between.
After final fold, divide dough in half or thirds, depending on desired size of baguettes. Gently round each piece into a rough ball shape and rest, covered with a towel, for 15 minutes.
Heat oven to 450°F. Turn each dough round so that any seams are facing up, and flatten into a rough rectangle. Fold the top third of the rectangle down toward the center and press the seam into the dough while gently pushing down and back, stretching the folded surface of the dough to create tension. Roll top third down once more, pressing seam and stretching surface of dough by pushing under the cylinder being created. Rotate dough 180°, fold remaining lip over the half-formed baguette, and seal by pressing down the length of the dough with the heel of your hand. Repeat once more, pulling dough into a uniform cylinder and sealing the seam to create tension. This video by Ciril Hitz illustrates the technique.
Using both hands, gently roll cylinder from center toward ends, stretching slightly with each pass. Properly rested and relaxed dough will extend easily, in 2 or 3 passes. Taper the ends slightly to seal; then set loaf seam-side-down on baking tray. Rest baguettes until they have doubled in bulk. Transfer to oven with a tray of hot water set under them. Bake 20 to 30 minutes, or until bread is dark golden brown and the loaves sound hollow when tapped on the bottom. If edges start to darken before bread is done, turn oven down slightly and continue baking until hollow sounding.