Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper; set aside.
For the pineapple BBQ sauce: In a small saucepan, combine the crushed pineapple, pineapple juice, brown sugar, soy sauce, garlic and red pepper. Bring to a simmer over medium heat and cook until the sugar dissolves, 5 to 8 minutes.
In a small bowl, stir together the cornstarch and 1 tablespoon water until smooth, then add to simmering sauce. Simmer until thickened, about 30 seconds. Remove from the heat and let cool.
For the pork: Place the pork tenderloins on a cutting board and make parallel incisions 1 inch apart along the length of the meat, most of the way through the meat. The bottom of the tenderloins should remain intact. Season with salt and pepper and place on the prepared baking sheet.
Put pineapple slices and red onion slices in the incisions, alternating between the two. (If necessary, trim the slices so they stick out about 1/4 inch.) Spoon 2 to 3 tablespoons of the sauce over each tenderloin and sprinkle with the jalapeno slices.
Bake until the internal temperature of the pork reaches 145 degrees F, 22 to 25 minutes. Let the meat rest for 5 minutes.
Transfer the tenderloins to a serving platter. Slice, garnish with scallions and serve with the remaining sauce.