For the vinaigrette:
Spread the greens over the base of a serving dish. Top with sliced fennel, grapefruit, and red onion and lightly toss. Immediately before serving, drizzle with vinaigrette. Top with crumbled cheese and fresh pepper.
For the vinaigrette:
Whisk together the red wine vinegar, mint, salt, and pepper in a medium-sized bowl. Slowly whisk in olive oil until emulsified. The vinaigrette can be made one day in advance and stored in the refrigerator if you do not add the mint until you are ready to serve the vinaigrette. Shake before using.