1 can (8 ounces) pineapple chunks in juice, drained (juice reserved)
2 tablespoons rice vinegar
2 tablespoons soy sauce
1 tablespoon cornstarch
Coarse salt and ground pepper
2 tablespoons vegetable oil
1 pork tenderloin (about 1 pound), halved lengthwise and thinly sliced
6 scallions, white and green parts separated, cut into 1-inch lengths
2 bell peppers (ribs and seeds removed), cut into 2-inch pieces
1 package (10 ounces) frozen broccoli florets, thawed and patted dry
rice, for serving
In a small bowl, stir together 1/3 cup pineapple juice, vinegar, soy sauce, cornstarch, and 1/4 cup water. Season sauce with salt and pepper.
In a large nonstick skillet, heat 1 tablespoon oil over medium-high. Cook pork, in two batches, until well browned on one side, about 1 minute (pork will cook more later); transfer to a plate.
To skillet, add scallion whites and bell peppers; season with salt and pepper. Cook, stirring, until peppers are crisp-tender, 6 to 8 minutes.
Add pork (along with any juices), broccoli, and pineapple chunks. Whisk sauce; add to skillet. Bring to a simmer; cook, stirring, until broccoli is warmed through, 2 to 4 minutes. Stir in scallion greens, and serve with rice.