Prep: 20 minutes ; Chill: 15 minutes; Cook: 35 minutes.
Preheat oven to 400°.
For the Cornmeal Tart Shell, lightly spoon cornmeal and flour into dry measuring cups; level with a knife. Combine cornmeal, flour, salt, and pepper in a food processor; pulse 4 times or until blended. Add butter and olive oil; pulse 10 seconds or until mixture resembles coarse meal. With processor on, slowly pour ice water through the food chute, processing just until blended. (Do not allow dough to form a ball.)
Press mixture gently into a 4-inch circle, and wrap tightly in plastic wrap. Chill dough for 15 minutes. Unwrap chilled dough, and place on piece of wax paper. Cover dough with a second sheet of wax paper. With a rolling pin, roll the dough into a 12-inch rectangle. Remove top sheet of wax paper; place dough, wax paper–side up in a 9- x 12-inch rectangular fluted tart pan. Remove remaining sheet of wax paper. Press dough gently into bottom and sides of tart pan. Cut away and discard excess dough.
Bake tart shell at 400° for 20 minutes. While the tart shell is warm, spread the goat cheese over the bottom of the crust in an even layer. Sprinkle evenly with thyme. Arrange the sliced tomatoes in an overlapping pattern on top.
Sprinkle sea salt and pepper evenly over tomatoes. Bake at 400°F for an additional 15 minutes. Allow tart to cool for 5 minutes before slicing. Garnish with thyme sprigs, if desired.