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Drain juice from pineapple into a large measuring C. (reserve tidbits for later use) and add water to measure 1 1/2 C.
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Add kosher salt, brown sugar and vanilla extract, stirring until salt and sugar dissolve.
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Cut tenderloin lengthwise about 3/4 of the way through, then open up and flatten slightly with hands (like a book) to butterfly.
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Place into a large zip-top plastic bag with the pineapple juice mixture; seal bag, turning to coat all surfaces (or use an oblong dish, covered with plastic wrap). Refrigerate for about an hour, turning occasionally.
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Preheat oven to 400°. Line a large rimmed baking sheet with foil. Remove pork from brine, discarding brine, and pat dry with paper towels. Place on prepared baking sheet; sprinkle evenly with lemon pepper and cook in preheated oven until an instant-read thermometer inserted in center of thickest part reads 155°, about 10-15 minutes.
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Remove to a cutting board; tent loosely with foil and let sit for 5 minutes, then cut lengthwise into quarters, then crosswise into bite-sized pieces.
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Place reserved pineapple tidbits into a large bowl with pork, cucumber, green onions and salad greens; add salad dressing, tossing to coat.
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Season as desired with salt and black pepper.
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To serve, line 4 serving plates with crisp lettuce leaves; top with pork mixture, sprinkle with diced tomato and serve immediately.