Halve the avocados lengthwise,peel and cut each half into 6 slices. Cut the cucumber thinly into 1/8" slices.

In a bowl, toss the avocado slices with 1/4 cup of the lemon juice and arrange the slices decoratively in layers in 1/2 of a shallow glass salad bowl. Mound the cucumber slices in the other half of the bowl and arrange the grated carrots around the edge opposite the avocado. In a small bowl combine the remaining 2 tbsp. of the lemon juice, salt, mustard. Add the oil in a stream, whisking and whisk the dressing until it is emulsified. Spoon the dressing over the salad and grind the pepper generously over the top. Serves 10.