Thinly slice the apples. Toss with the lemon juice in a large bowl.
Make the dressing: Heat the olive oil in a skillet over medium heat. Add the shallots and 1/2 teaspoon salt and cook, stirring occasionally, until golden, about 8 minutes. Whisk in the vinegar and mustard; remove from the heat and let cool slightly. Whisk in the sour cream and 1/4 cup water.
Add the dressing to the bowl with the apples. Add the ham, watercress and endives. Season with salt and pepper and toss to coat.