Cut the top off the fennel bulb, saving the fronds for garnish. Discard the stems. Trim the bottom of the fennel, quarter the bulb, and trim the tough core in the center. Thinly slice the fennel and place in a glass bowl.
Add the red onion to the fennel, along with the vinegar. Toss to evenly coat and let it sit for about 20 minutes.
Prepare the oranges by cutting the tops and bottoms off and placing it on one of the cut sides on the cutting board. Use a sharp knife to follow the curve of the orange and slice off the peel and any white pith. Turn the orange on the side and cut into 1/4 inch rounds. Arrange on a platter.
Arrange the fennel and red onions on top of the orange slices, reserving about 8 slices of red onion and the liquid in the bowl.
Finely chop the reserved red onion and return to the reserved vinegar. Whisk in the olive oil, salt, and pepper.
Right before serving, arrange the avocado on top of the salad. Drizzle the dressing on top. Garnish with fennel fronds and mint, if using. Sprinkle with good sea salt, and enjoy!