Thinly slice the fennel bulb and reserve the fronds for garnish. Peel and segment the pink grapefruits, ensuring to remove all the membrane. Peel the avocado and cut it into bite-sized chunks. Rinse your lightweight greens under cold water and pat dry.

In a large mixing bowl, combine the fennel, avocado chunks, grapefruit segments, and lightweight greens. Gently toss everything together to distribute evenly.

In a medium jar, combine the grapefruit juice, olive oil, white balsamic vinegar, minced shallot, and brown mustard. Add salt and pepper to taste. Secure the lid and shake vigorously for about 30 seconds.

Drizzle the citrus dressing over the salad mixture. Use tongs or a large spoon to gently toss until all ingredients are coated.

Transfer the salad to a serving platter and top with shaved Parmesan-Reggiano and reserved fennel fronds. Enjoy!