• Place a foil-lined baking sheet on top rack (for 4 servings, use 2 foil-lined sheets on top and middle racks) and preheat oven to 450 degrees. Wash and dry produce. • Halve, peel, and thinly slice onion. Drain pineapple over a small bowl, reserving juice. Stir 1⁄4 of the onion, half the vinegar (you’ll use the rest later), and a pinch of salt into bowl with juice. Set aside.

• Heat a drizzle of oil in a large pan over medium heat. Add remaining onion; cook, stirring and adding splashes of water as needed to prevent sticking, until softened, 6-8 minutes. While onion cooks, rinse shrimp under cold water. Pat shrimp or chicken dry with paper towels and season all over with salt and pepper. Heat a drizzle of oil in a second large pan over medium-high heat. Add shrimp or chicken in a single layer; cook, stirring occasionally, until cooked through, 3-5 minutes for shrimp or 4-6 minutes for chicken. Transfer to a plate.

• While onion cooks, halve, core, and very thinly slice green pepper into strips. Toss in a second small bowl with a drizzle of olive oil, salt, and pepper. Dice mozzarella into 1⁄2-inch pieces. Finely chop cilantro.

• Once cooked onion is softened, increase heat under pan to medium high. Add pineapple and a drizzle of oil; cook, stirring, until pineapple is lightly browned, 2-3 minutes. • Stir in remaining vinegar. Cook, stirring, until onion and pineapple are caramelized and jammy, 1-2 minutes more. Turn off heat and season with salt and pepper.

• Brush or rub each flatbread with a drizzle of oil. Carefully remove prepared baking sheet from oven; place flatbreads on sheet. (For 4 servings, divide flatbreads between prepared sheets.) • Evenly spread flatbreads with BBQ sauce, then top with caramelized onion and pineapple, green pepper, Monterey Jack, and mozzarella. Top flatbreads with shrimp or chicken after adding mozzarella.

• Return flatbreads to top rack until cheese melts and edges are golden brown, 10-12 minutes. (For 4 servings, return to top and middle racks; swap rack positions halfway through baking.) • Transfer flatbreads to a cutting board and top with cilantro and pickled onion (draining first) to taste. Cut into pieces, divide between plates, and serve.