Segment the grapefruits: Trim ends, cut away peel and pith, then cut between membranes to release segments over a bowl to catch juices. Squeeze the leftover membranes to collect 0.25 cup grapefruit juice.
Make the dressing: In a small bowl, whisk the extra-virgin olive oil, 0.25 cup reserved grapefruit juice, lemon juice, Dijon mustard, honey, kosher salt, and black pepper until emulsified.
In a large bowl, combine the mixed baby greens and red onion. Drizzle with about two-thirds of the dressing and toss gently to coat.
Halve, pit, and peel the avocados, then slice. Add the avocado and grapefruit segments to the dressed greens. Spoon over the remaining dressing and gently fold just to combine without crushing the avocado.