How to Make It
In a medium saucepan, simmer the maple syrup over moderate heat until beginning to thicken slightly and darken in color, about 7 minutes. Whisk in the stout and vinegar and bring to a boil. Simmer over moderate heat, stirring occasionally, until the glaze is thickened and reduced to 3/4 cup, about 12 minutes.
Light a grill. In a large bowl, toss the wings with olive oil and the salt, black pepper and cayenne. Grill the wings over moderately high heat, turning occasionally, until lightly charred and just cooked through, about 15 minutes.
Transfer the chicken wings to a large clean bowl and toss with half of the stout glaze. Return the wings to the grill and cook, turning once, just until sticky and caramelized, about 2 minutes. Return the wings to the bowl, toss with the remaining stout glaze and grill, turning once, until caramelized, 1 to 2 minutes longer. Transfer the wings to a serving platter and garnish with flaxseeds and scallion.