To make dressing, combine lemon juice and flavored salt in a jar. Cover with the lid and shake to blend. Add the oil and shake to blend. (You can also whisk all the ingredients together in a small bowl.)
With a mandoline or sharp knife, slice the zucchini lengthwise as thin as possible. Arrange the slices on a platter and pour the dressing over them.
Tilt the platter back and forth to coat the slices evenly.
Cover with plastic wrap and let marinate at room temperature for 30 minutes, so the zucchini absorbs the dressing and does not dry out.
Halve, pit and peel the avocado, and cut it lengthwise into very thin slices.
Carefully arrange the slices of marinated zucchini on individual salad plates, alternating with the avocado slices, slightly overlapping them.
Sprinkle with the pistachio nuts.
Garnish with the thyme sprigs an flowers and fleur de sel.