Using a mandoline, slice the fennel into thin shavings.
Cut the bottom and top off of the grapefruit. Using a knife, cut the peel off of the sides, following the curve of the grapefruit and being careful to only cut away the peel and bitter pith. Hold the fruit in one hand over a bowl and cut the flesh of the grapefruit away from the membrane to release a wedge. Repeat until all the segments are released.
In a large saute pan over medium heat, coat with olive oil and add the fennel. Season with salt and saute until the fennel is warm and coated with the oil, 1 to 2 minutes. Remove from the heat. Toss in the grapefruit segments, olive slivers, and juice and sprinkle with the fennel fronds. Serve immediately.