In a medium saucepan over medium heat, melt the butter. Add the onions and rhubarb and cook until softened, about 8 minutes. Add the pomegranate juice, honey, soy sauce, red pepper flakes and a pinch of salt and pepper. Simmer until the liquid is reduced by half, 6 to 8 minutes. Transfer to a blender and puree. Strain and reserve.
Heat a grill pan or grill over medium heat. Spread the spring onions in a single layer on a sheet pan. Drizzle with 1 tablespoon of the oil and sprinkle with smoked salt and pepper; toss to coat. Grill the spring onions, turning occasionally, until tender and charred in spots, 10 to 15 minutes.
Rub the pork with salt and pepper and the remaining tablespoon oil. Place the pork on a sheet pan fitted with a rack. Roast the pork for 10 minutes, then reduce the oven temperature to 325 degrees F.
Brush the pork on all sides with the glaze and return to the oven. Continue to roast, repeating the basting twice, until the internal temperature reaches 145 degrees F, about 40 minutes. Baste again when it comes out of the oven.
Let the pork rest for 10 minutes. Slice the pork into medallions. Brush some glaze on the plate and arrange the medallions on the glaze. Serve with the onions. Garnish with the almonds, mint and more smoked salt.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.