Make vinaigrette: In a blender, whirl marmalade and verjus together. With blender running, drizzle in oil; add salt and pepper. Chill until ready to use. "This dressing keeps for 3 weeks at least, because there's nothing in it that will change flavor, like garlic or fresh herbs or shallots."
Make salad: With a sharp knife, slice peel and pith from oranges, cut them crosswise into wheels, and remove seeds. Slice avocado and season with salt and pepper. "People don't season their salads enough. The ingredients need salt too, especially the watery ones." Toss endive and watercress with about 1/4 cup dressing.
Arrange oranges, avocados, and salad to one side of 4 rectangular plates. Set a chicken leg on each plate, next to salad. Crumble about 1 oz. of cheese onto each salad, and drizzle with about 1 tbsp. more dressing.
*Find verjus (unfermented grape juice) at well-stocked grocery stores or online.