Special equipment: Caja China grill
For the pickled grilled green onions: Heat a charcoal or gas grill to high for direct grilling.
Brush the green onions with canola oil and season with salt and pepper. Grill until charred and just wilted, turning once.
Combine the vinegar, 3/4 cup water, the sugar, 1 tablespoon kosher salt, the coriander seeds, garlic and jalapeno in a medium nonreactive saucepan, bring to a boil and cook until the sugar has melted. Remove from the heat, transfer to a nonreactive bowl, add the green onions, cover and refrigerate for at least 2 hours and up to 2 days.
For the duck sliders: Prepare the Caja China according to the manufacturer's directions. Make some balls out of aluminum foil to prop up the Caja China rack that sits over the drip pan--the rack should be slightly elevated in the drip pan so that the ducks stay above the fat that will render into the pan during the cooking process. Put the rack on the foil in the pan.
Brush the ducks on both sides with canola oil and sprinkle with the ancho powder and some salt and pepper. Put the ducks in breast side-down on the rack, put the cover on and put the hot coals on top. Add about a chimney full of cold coals to the hot--they will catch and cook as the hot coals burn. Add more cold coals as needed to keep the fire hot throughout the cooking process. Cook the ducks for 45 minutes.
Remove the coals and lid, brush the backs of the ducks with some of the barbecue sauce, flip over, brush the tops with more of the sauce and continue cooking until golden brown and cooked through and the skin is crispy, about 45 minutes more.
While the ducks cook, combine the remaining barbecue sauce, the apple juice concentrate, sugar, fennel, cinnamon, star anise and habanero in a medium saucepan and cook until heated through and slightly reduced, about 30 minutes; stir in the cilantro. Remove 1/4 cup of the sauce and set aside until cool enough to add to the creme fraiche.
Remove the ducks to a cutting board, let rest for 15 minutes and shred into bite-sized pieces. Put the meat in a bowl and toss with the sauce.
Whisk together the creme fraiche, cooled reserved sauce and the lime zest and juice in a medium bowl.
To assemble, spread some of the creme fraiche mixture on the top and bottom buns. Mound some of the duck on the bottom of the buns, top with some of the pickled green onions, put the bun tops on top and enjoy.