In the bowl of a food processor combine the flour, cornmeal, salt and pepper. Pulse once or twice to combine. Add the diced butter and pulse until you end up with small pieces of butter, similar to the size of rice. Add 1/4 cup ice water to the dough and pulse until it comes together. If needed add 1-2 tablespoons more ice water until the dough comes together. Turn the dough onto a floured work surface, knead a couple of times and then form the dough into a flat disk. Wrap in plastic wrap and refrigerate for at least 30 minutes.
Meanwhile, add the butter to a skillet set over medium-high heat. Add the onions and 1/4 teaspoon each of salt and pepper. Cook, stirring constantly, until the onions begin to soften, about 5 minutes. Continue to cook, scraping the browned bits off the bottom of the pan frequently until the onions are golden brown, about 20 minutes. Remove from the heat and stir in the thyme. Season lightly with salt and pepper.
Now grab your dough from the fridge. Flour your work surface and roll the dough to about 1/8-inch thickness. Transfer to a baking sheet lined with parchment paper.
Crumble the blue cheese over the bottom of the dough leaving a 3-inch border around the edges. Sprinkle on the shredded cheddar. Add the caramelized onions in an even layer and then layer on the basil, zucchini, sprinkling it with salt + pepper. Now add the tomato slices and half of the cherry tomatoes, reserving the remaining half for topping. Drizzle the tomatoes with olive oil, sprinkle with salt and pepper. Now drizzle lightly with a little honey. Fold the edge of the dough over the tomatoes. Brush the crust with the beaten egg. Place the galette in the fridge for 15 minutes or until ready to bake.
Preheat the oven to 375 degrees F.
Bake the galette for 45-55 minutes or until the crust is golden and the tomatoes are very lightly charred. Allow to cool 5 minutes and then slice and serve topped with fresh cherry tomatoes and a drizzle of honey, if desired. The crust will be crumbly so cut small slices for easier serving.