To make the salad, wash the fennel and discard the tough outer bulb. Reserve some feathery leaves or fennel fronds for garnish.
Cut the bulb in half and use a mandoline to thinly shave the fennel and place in a bowl with cold water.
Cut a slice off the top and bottom of each grapefruit so they have a flat bottom to sit on. Use a sharp knife to cut the peel and remove as much of the pith from the grapefruits as you can. Segment the grapefruits into a prep bowl. Reserve grapefruit juice for dressing.
To make the dressing, in a sauce bowl, whisk together olive oil, grapefruit juice, mustard, and honey. Season to taste with salt and pepper.
Bring a pot of water to boil over medium high heat, add the rice and cook according to package instructions. Rinse under cold water, drain, then set aside to cool.
Lay the scallops on a paper towel and gently blot to remove any excess moisture. Season the scallops on both sides with salt and pepper.
Heat a large cast iron pan on medium-high heat until very hot. Add half the olive oil and heat until the surface is rippling but not smoking. Pour off any excess oil leaving only a thin, even film of oil on the pan.
Add half the scallops to the pan and they should audibly sizzle as soon as they hit the pan. Press down firmly with a spatula to ensure even browning. Sear the scallops for a minute and a half until they develop a nice, golden crust.
Flip the scallops and press down again with the spatula and cook for another 45 seconds to a minute, depending on the thickness of the scallops.
When the scallops are cooked, remove them from heat and set them on a plate. When you cut into the scallop, it should be translucent in the middle.
Repeat the step with the remaining scallops.
When ready to serve, add the rice, drained fennel slices, and segmented grapefruits to a serving platter. Arrange the seared scallops on top. Garnish with fennel fronds. Drizzle with some of the pan juices and salad dressing.
Serve this salad for a casual dinner or serve smaller portions as an elegant starter or first course for six to eight people.