10cups all-purpose flour, plus extra for flouring work surface
2medium zucchini, cut lengthwise into 1/4-inch slices
3green, red or yellow bell pepper, seeded and cut into large chunks
1red onion, sliced 1/4-inch thick
To prepare olives, combine all ingredients in a small bowl. Let stand at room temperature until ready to use in panini.
To prepare flatbread, combine all ingredients in a large bowl. Mix until dry ingredients are incorporated. Cover with a damp towel and set aside 2 hours. If you’re making the dough to use later, remove the damp towel, lightly flour the surface of the dough and push it down with your hands, deflating the mix back to near its original volume. Seal container with a lid and store in your refrigerator up to 2 weeks.
When you’re ready to use the dough, heat grill. Lightly flour your hands, work surface and rolling pin. Pinch 4 (2- to 3-inch) balls of dough from the mix. Replace the lid and return the rest to the fridge. Place 1 dough ball on the work surface and pat into a flat disk. Use a rolling pin to roll to desired thickness and set aside on floured parchment or wax paper. Repeat with remaining dough balls.
Use a pizza peel or flat cookie sheet to transfer the rounds to the grill. Cook 3 to 5 minutes, flip dough halfway through the cooking time, until the dough is cooked through and bubbly.
To prepare vegetables, place all vegetables in a large bowl. Toss with oil, salt and pepper. Place vegetables on grill grate and cook 3 to 5 minutes, flipping halfway through the cooking time, until vegetables are tender and slightly charred.
To assemble panini, olives and vegetables on top. Fold over bread and grill until thoroughly heated.