Place the juniper berries and coriander seeds in a resealable freezer bag. Using a mallet or rolling pin, coarsely crush the seeds. Transfer to a small saucepan; add the gin and jam. Bring to a boil over medium-high, stirring often, until reduced to about 3 tbsp., 3 to 5 minutes. Strain the glaze into a small bowl.
In a large bowl, mix the cabbage, cilantro and scallion. Toss with the red wine vinegar and 4 tsp. glaze; season the slaw with salt and pepper.
Pat the mahi mahi dry; season. In a medium nonstick skillet, heat the oil over medium-high. Cook the fish, skin side down, until browned, 2 to 3 minutes per side, adding the remaining glaze to the skillet during the last minute of cooking.
Divide the slaw between plates. Arrange the mahi mahi alongside and drizzle with the pan juices.