Slice the tops and bottoms off of grapefruits. Stand each one on its end on a cutting board, and slice off the peel, cutting from top to bottom to expose the flesh. Cut along the edge of each segment to separate the segments from the membrane, and place the segments in a large bowl, along with any juice that remains.
In a small bowl, combine the oil, lime juice, a pinch of salt, and 2-3 tablespoons of juice from the bowl of grapefruit segments. Whisk well to incorporate.
Add the fennel and mint to the bowl of grapefruit and toss to combine. Drizzle with the dressing, and toss well. Gently fold in the sliced avocado and sprinkle with cracked coriander, if using. Season to taste with salt and pepper, garnish with fennel fronds, and serve.