In a food processor, combine the flour, cornmeal, salt, and pepper. Pulse a couple times to mix. Scatter the cold butter cubes over the flour mixture and pulse until the butter is broken down into pea-sized pieces, about 10 to 12 pulses.

Add 1/4 cup ice water and pulse until the dough just starts to clump together. If it seems dry, add 1 to 2 tablespoons more ice water, pulsing after each addition. Turn the dough out onto a lightly floured surface, knead it gently a few times to bring it together, then shape into a flat disk. Wrap tightly in plastic wrap and refrigerate for at least 30 minutes, or up to 2 days.

While the dough chills, melt the 2 tablespoons butter in a large skillet over medium-high heat. Add the sliced onion and 1/4 teaspoon each of salt and pepper. Cook, stirring often, until the onion softens, about 5 minutes.

Reduce the heat to medium-low and continue cooking, scraping up any browned bits from the bottom of the pan, until the onions are deep golden brown and very tender, about 20 minutes. Stir in the thyme leaves, then season with additional salt and pepper to taste. Remove from heat and set aside.

Preheat the oven to 375°F. Line a large baking sheet with parchment paper. On a lightly floured surface, roll the chilled dough into a 14-inch round about 1/8-inch thick. Carefully transfer the dough to the prepared baking sheet.

Sprinkle the crumbled blue cheese evenly over the dough, leaving a 3-inch border around the edges. Top with the shredded cheddar, then spread the caramelized onions in an even layer. Arrange the torn basil leaves over the onions, followed by the zucchini rounds in a single layer. Season lightly with salt and pepper.

Layer the heirloom tomato slices over the zucchini, then scatter half of the cherry tomatoes on top. Drizzle everything with olive oil, season again with salt and pepper, and drizzle with a little honey. Fold the exposed dough border up and over the filling, pleating as needed. Brush the folded crust with the beaten egg.

Place the assembled galette in the refrigerator for 15 minutes to firm up the crust before baking.

Bake the galette for 45 to 55 minutes, until the crust is deep golden brown and the tomatoes are lightly charred at the edges. Let it cool on the baking sheet for 5 minutes.

Slice into wedges and serve warm or at room temperature. Top with the reserved cherry tomatoes, an extra drizzle of honey, and a sprinkle of toasted walnuts and fresh thyme, if desired.