Heat butter and 1 tablespoon oil in a large saute pan over medium high heat. Pat scallops dry and add to the hot pan; sear until caramelized, 6 to 7 minutes, flipping halfway.
While scallops are cooking, peel, skin, and slice grapefruit. Slice fennel thinly, reserving a few fronds. Place grapefruit, fennel, fronds, and spinach in a large bowl.
Prepare dressing by mixing remaining 2 tablespoons olive oil, vinegar, lemon juice, shallot, mustard, honey, salt, and pepper together in a small bowl.
Add seared scallops to the salad and mix to combine. Drizzle dressing over top.
Seared Scallops with Grapefruit-Fennel Salad · Dinner Match Lab | Dinner Match Lab