Just a few simple ingredients - heirloom tomatoes, ricotta and parmesan cheeses, and fresh herbs - are pure culinary magic in this simple galette recipe. The result is substantial enough for a meal along with a tossed green salad.
Make the crust. Combine flour, cornmeal, and salt in a large bowl and whisk until combined. Add the butter, then crumble into the flours with your fingers or a pastry cutter until the mixture is coarse and the butter pieces are no larger than peas.
Separate the egg and set aside the whites to use later. Add the egg yolk and 4 tbsp ice water to the dry ingredients. Use your hands to work the dough until it just comes together and isn't crumbly, adding more water if needed. The finished dough should be slightly tacky. Form into a disk, wrap in plastic, and chill at least 30 minutes or up to 2 days.
When you're ready to bake, heat the oven to 400°F.
Prep the tomatoes. While the oven heats, prepare the tomatoes. Lay the tomato slices onto a cutting board and sprinkle with salt. Rest for 10-15 minutes, or until the tomatoes weep a portion of their water content. Pat the tomatoes dry with paper towels.
Form the galette. Roll out the chilled dough on floured parchment paper to a scant 1/2" thickness into a roughly 12" circle. Transfer the dough using the parchment paper to a baking sheet.
Scatter parmesan across the dough, leaving a 1-2" border, then dot with the ricotta cheese. Sprinkle the cheeses with half the fresh thyme.
Layer the tomatoes on top of the cheeses. Sprinkle the remaining thyme, cracked black pepper, and additional salt on top of the tomatoes. Fold up crust around the filling, crimping the edges together, into a pie shape. Whisk the egg whites until smooth, then brush the crust with the whites.
Bake the galette. Bake 40-45 minutes until the crust is golden brown. Let stand for 5-10 minutes and then slice and serve. Enjoy warm or room temperature. Leftovers keep in a sealed container in the refrigerator for up to three days.