How can I improve the shape/appearance of this bread?

I've been making bread from this recipe:
550g flour
370 water
12g kosher salt
4g yeast
Knead in the stand mixer, shape into an ovoid bâtard kind of deal,
slash diagonally, and bake 45 min. at 450F/230C.
Taste & texture are good, but instead of a nice-looking loaf with open slash marks, I get the misshapen broken-backed loaf in the photos below. How can I produce a more normal-looking loaf?
EDIT: I've made this recipe a bunch of times and the results are very consistent.
Best Answer
Before we start on what might have gone wrong, let me assure you that you obviously are doing a lot of things right:
You have surface tension in your loaf and good oven spring, meaning you got the shaping right and seem to put your bread in the oven at a good time during the final rise - my guess would be "slightly underproofed", which is exactly what you want for a wheat bread like this one.
With your slashes I suggest that you dump the X-acto, because the blade is too short. The classic lamé is one possible route (and can be easily substituted by an oldfashioned razor blade on a chopstick), but you probably need a few more loaves to get the hang of using it - you need to work fast and the bent blade and thin handle is different from what you are used from kitchen knives. A serrated knife is a viable alternative, provided it is quite sharp. Whatever you use, you don't want to use a "sawing" or "zig-zaging" motion, but just one quick swiping movement per slash.
The higher the hydration the more likely is your slash to seal itself again, simply by the weight of the dough. Therefore an almost horizontal slash is recommended. The idea is to create a "flap", not a "valley". The thin flap will become the "ear" as the two overlapping parts drift apart due to oven spring. For a loaf like yours, I personally aim for a cut of almost an inch. This is no hard and fast rule, every dough and bread type is different. Once you get the basic technique right, you can experiment.
Note that the bread will expand perpendicular to the cut, so diagonal cuts should be always oriented mostly along the long axis and multiple cuts should be running parallel for quite a bit. More crosswise cuts will create the "bulge" that can be seen on your loaf, almost as if the loaf "arches its back".
Good steaming will keep the crust pliable, so that oven spring can open the slashes well.
Pictures about "How can I improve the shape/appearance of this bread?"



How can I make my bread look better?
Dusting with flour gives a rustic look to the loaf. Egg wash turns the bread golden and gives a softer crust. Milk washes in the last few minutes are good for a sandwich style loaf and give a glossy brown, soft crust. Brush loaves or rolls with oil or water and roll in seeds or grains to coat before baking.How do I shape my bread?
Shaping is a process that allows the gluten to develop and creates surface tension by forming a skin on the surface of the dough. This skin helps the dough to create a crispy, even exterior during the baking process to prevent it from collapsing or breaking apart in the oven.How to shape bread
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Answer 2
I'm not much of an expert, but it looks to me like you should be changing the direction of your scorin as this guide recommends; for batards, you want to slash more top-to-bottom instead of across the loaf. Also, angle the cut at about 30 degrees, so you're cutting more of a flap than a gash.
Also, I don't know how you're shaping it before the prove, but here is a guide that might help?
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Images: SHVETS production, Karolina Grabowska, Antonio Prado, Alex Green