Here's something probably a thousand people on this site have figured out before me, but just in case someone hasn't thought of this yet:
The problem: A 12 inch diameter dutch oven. Handles boules quite nicely. Still, you don't always want the boule shape. The maximum size for a batard would be 12 inches of course. Unless you don't mind a circular batard, which allows you (pi X diameter) or 3.14 X 12 inches = 37.7 inches for your batard length, or at least external circumference. Now that's a substantial batard.
I kept it simple and used 1,000 gm of KA bread flour, 68% hydration, 2.2% salt, and 0.08% (0.8 gm) instant yeast with overnight bulk fermentation. This follows Ken Forkish's formula for "weeknight white bread" on page 91 of "Flour, Water, Salt, Yeast". I divided the dough in two, formed two normal batards, then kind of bent them together, making sure the outside diameter stayed at about 11 inches, to insure an easy drop into the super hot dutch oven.
I painted a little extra virgin olive oil on the top to try and prevent "skin" formation while the thing proofed for 90 minutes uncovered. Baked at 475 degrees with lid on for 25 minutes and then 380 degrees with lid off for 30 minutes.
Here are the results:
Won't know how the crumb looks until friends come over for dinner tonight. I took care to bake until bottom was at the verge of having a few dark brown to black areas in order to make sure the crumb wasn't too doughy. I have a good feeling about it though. My friends will eat anything, especially if an adequate supply of lager and Malbec is at hand. My wife made a very nice beef borguignon (spelling?) for the main event.
Best to all,