Meanwhile, toss the mushrooms with 2 tablespoons of the lemon juice and 1/2 teaspoon of the salt in a large bowl. Whisk the remaining lemon juice with the mustard, remaining salt, and pepper, to taste, in a small bowl. Gradually whisk in the olive oil, starting with a few drops and then adding the rest in a steady stream to make a smooth, slightly thick vinaigrette. Add the shallots.