Whisk the lemon juice, honey, Dijon, salt and pepper in a small bowl, then stream in the olive oil while whisking until the dressing is smooth and slightly thickened. Set aside.
Cut the peel and white pith from the oranges and grapefruit. Slice the oranges into 1/4-inch rounds. Segment the grapefruit over a bowl to catch the juice and reserve 1 tbsp of it to stir into the dressing if you like.
Soak the sliced red onion in a bowl of cold water for 5 minutes, then drain and pat dry; this takes the sharp edge off.
Spread the arugula over a wide platter or shallow bowl and arrange the orange rounds and grapefruit segments on top.
Scatter the red onion and pomegranate arils over the fruit, then drizzle with the dressing.
Crumble the goat cheese over the salad, sprinkle with pistachios, and serve right away.