5 Tbsp. extra-virgin olive oil, for greasing and drizzling
2 delicata squash (about 2 lb. total)
1 large onion, peeled, cut into thin rounds
3–4 sprigs thyme, leaves stripped
Freshly ground black pepper
½ cup crumbled mild blue cheese (such as Danish Blue or Cambozola)
2 cups arugula
2 tsp. finely grated lemon zest
1 Tbsp. fresh lemon juice
Flakey sea salt to finish
Directions
If using a dough: grease a large rimmed baking sheet with 3 Tbsp. oil. Turn dough out onto a floured work surface and roll to a 16x11" rectangle. Using rolling pin, roll up dough and transfer to prepared sheet. Using your fingertips, spread dough to edges of pan. Cover dough with a kitchen towel and let rest 30 minutes. (If using a premade crust or a flatbread, skip this step.)
Preheat oven to 400°. Halve squash lengthwise, scrape out seeds, and slice crosswise into ¼"-thick half-moons. Transfer to a large bowl. Add onion, thyme leaves, and 2 Tbsp. oil; season with salt and pepper.
Scatter squash mixture over dough. Drizzle oil over. Bake flatbread until crust is browned and cooked through and vegetables are light golden brown and tender, about 45 minutes. Transfer baking sheet to a wire rack and sprinkle blue cheese over while flatbread is still warm.
Just before serving, toss arugula, lemon zest, and lemon juice in a small bowl. Drizzle with oil; season with salt and pepper. Top flatbread with arugula salad.