Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.

Halve, peel, and thinly slice onion. Trim and thinly slice scallions, separating whites from greens.

Open package of chicken* and drain off any excess liquid.

Heat a drizzle of oil in a large pan over medium-high heat. Add chicken; season with Fry Seasoning, salt, and pepper. Cook, stirring occasionally, until beginning to brown, 2-4 minutes.

Add onion and scallion whites; cook, stirring occasionally, until onion has softened and chicken is cooked through, 2-4 minutes more. Season with salt and pepper.

Meanwhile, place flatbreads, dimpled sides down, on a baking sheet (divide between two baking sheets for 4 servings). Brush or rub each flatbread with a drizzle of olive oil; lightly season with salt and pepper.

Toast on top rack until golden brown, 3-5 minutes. (For 4, toast on top and middle racks, swapping rack positions halfway through.)

Once flatbreads are toasted, remove sheet from oven. Carefully flip dimpled sides up; spread evenly with mustard. Sprinkle with gouda and mozzarella. Top with chicken mixture. Return to top rack; bake until cheese melts and flatbreads are crispy, 3-4 minutes. (For 4, bake on top and middle racks, swapping rack positions halfway through.)