Heat oven to 325 degrees. Line a sheet pan with parchment paper. Pat the salmon dry, then place on the tray skin-side down and season with salt and pepper.
In a medium bowl, stir together the dill, ginger and olive oil until combined. Season with salt and pepper. Spread half of the dill-ginger mixture over the top of the salmon. Bake until cooked through, 15 to 20 minutes.
As the salmon cooks, cut off the top and bottom of the grapefruit and peel away the skin. Squeeze the peels into the remaining dill-ginger mixture to get out any juice. Cut the fruit in half and slice into half-moons, removing the seeds.
Transfer the fruit and any juice to the bowl. Repeat with the oranges. Add the radishes, season with salt, and stir gently to combine.
Break the salmon into large pieces, and divide across plates with the citrus salad. Add the avocado, season with salt, and spoon the juices over top.
