FOR THE RHUBARB JAM: In a small pot, combine the rhubarb, sugar, water, orange zest and orange juice. Bring to a boil, then reduce to a gentle simmer and cook, stirring occasionally, until rhubarb is broken down and thickened, and liquid has been absorbed, about 45 minutes. Remove from heat and set aside to cool. It will thicken more as it cools.

FOR THE CARAMELIZED ONIONS: Meanwhile, in a large saucepan over medium heat, melt the butter. Add the onions, a few handfuls at a time, and cook, stirring often, until translucent. Reduce heat to low and cook slowly until onions are very soft and a deep golden brown, about 45 minutes. If onions start to stick, add a splash of wine or water and stir to absorb. Set aside to cool.

FOR THE FLATBREAD: Heat the oven to 400 degrees F. Place the pizza dough on a rimmed baking sheet and drizzle with oil, tuning to coat the dough lightly. Using clean hands, gently stretch the dough to spread it as evenly as possible towards the four corners of the sheet. If it springs back, give it a few minutes to rest and try again, until the dough covers as much of the baking sheet as possible without tearing.

Sprinkle the dough with salt and pepper and bake until beginning to crisp around the edges, about 10 to 12 minutes. Remove flatbread from the oven and top with an even layer of the caramelized onions. Sprinkle the goat cheese all over and scatter the prosciutto pieces. Return the flatbread to the oven and bake until the dough is golden and the prosciutto is crispy, 10 to 15 minutes more.

Transfer the flatbread to a cutting board. Dollop with the rhubarb jam, and sprinkle with chives and lemon zest to serve.

Note: any remaining rhubarb jam can be refrigerated in an air-tight container for about two weeks (it’s delicious with yogurt, granola, over ice cream or on toast!).