Preheat oven to 375 degrees. Combine sliced fennel and red onion on a medium rimmed baking sheet. Drizzle with 2 tablespoons olive oil, season generously with sea salt and pepper; and toss to combine. Roast until fennel is tender and deeply browned in spots, 25–30 minutes.
Meanwhile, cut the ends off the grapefruit to reveal flesh. Place one cut side on a cutting board and whittle away the skin, leaving as much fruit as you can behind.
Carefully cut out the segments, as close to the membrane as possible and place in a medium bowl. Squeeze any juice from the membrane into the bowl; add red wine vinegar and season with sea salt and pepper. Set aside. Coarsely chop reserved fennel fronds.
Heat a large skillet over medium heat. When pan is hot but not smoking, place sausages in pan. Sear for 2-3 minutes on each side until slightly browned. (Sausages are pre-cooked, so don’t overcook them or they will dry out.)
When fennel is done, transfer to a serving platter. Place sausages on top and scatter with grapefruit segments and fennel fronds; and serve.