To prepare the marinade for the pork, combine 3 tablespoons oil, 1-2 sliced scallions, garlic, chiles, soy sauce, 1/2 of the lemon juice, 1 tablespoon brown sugar, allspice, salt, pepper, thyme, nutmeg, and ginger in a food processor. Pulse until you reach a somewhat smooth texture. (it's okay if it's a little chunky) Pour the marinade in a large plastic freezer bag and add the pork loins. Turn them to coat and then seal the bag, rolling it into a tight cylinder. Place in a medium bowl (to prevent leaking) and let marinate in fridge for about 6 to 8 hours.
Light a grill under medium high heat. Take the pork out of the marinade and brush with remaining tablespoon of oil and season with salt and pepper. Grill the pork over indirect heat until internal temperature is about 135 F, about 20-30 minutes, depending on your grill. Let rest on cutting board for about 10 minutes, then slice into thin medallions.
While you're waiting for the pork to rest, you can quickly whip up the pineapple jicama salsa. In a medium bowl, combine the onion and the remaining lemon juice. Let sit for about 5-10 minutes so the onion can soak up the lemon flavor. Add in the pineapple, pineapple juice, jicama, cilantro, remaining brown sugar, jalapeno, and remaining scallions. Toss quickly and serve with sliced pork.