Serves 4; can be multiplied.
For the vinaigrette:
Juice of 1/2 orange (approximately 2 tsp)
1 tsp Dijon mustard (I like grainy mustard, but use what you have)
1/8 tsp kosher salt
1/4 tsp fresh black pepper
2 tsp extra virgin olive oil
For the salad:
1 large fennel bulb
1 ripe avocado
1 cup grapefruit segments
In a small jar with a tight-fitting lid, combine all of the vinaigrette ingredients. Shake vigorously, to emulsify the dressing.
Trim the fennel bulb by cutting across the top to remove the stalks (save the green fronds), and slicing across the bottom at the root end. Cut the bulb in half lengthwise, and remove the hard core from each half.
Use a mandoline, if you have one, to create paper-thin slices of fennel. Or, slice as thinly as you can with a very sharp knife. Peel the avocado, and cut into slices. Add them to a bowl with the fennel and grapefruit segments.
Immediately toss with the vinaigrette, making sure to distribute the dressing over all of the avocado and fennel. Set aside at room temperature for 30-60 minutes. The longer the salad "marinates", the more tender the fennel will be.
Just before serving, toss some of the green fennel fronds into the salad.