- Make the agrodolce: In a large, heavy-bottomed pot, render bacon until crispy. Add onions and caramelize until medium-dark brown. Deglaze pan with apple cider vinegar, making sure to scrape bottom of pan. Add collards and cook down until tender, about 45 minutes to 1 hour. Add balsamic, honey, and chili flakes. Let reduce 10 to 15 minutes and reserve hot in pot until ready.
- Grill the meat: Heat grill and season loin heavily with salt and pepper. On hottest part of grill, sear loin for 8 minutes total, giving it a quarter-turn every 2 minutes. Move to cooler side of grill and let slowly cook to medium, 145 to 150 degrees on a meat thermometer. Let rest for 4 to 5 minutes. Slice thin.
- Spread agrodolce across a large platter, top with loin, and garnish with red onion.