In a small bowl, whisk together the vinegar, mustard, salt, pepper, and olive oil.

Just before serving, assemble all of the ingredients for the salad on the counter. Turn on a stovetop burner to medium-high. Set a large stainless steel bowl over the burner and heat until very hot. Using an oven mitt or pot holder to protect your hands, pour the vinaigrette into the bowl and swirl for a minute or two, until the vinaigrette is hot. Quickly add the spinach and shallots and toss the salad with tongs to coat evenly. When the spinach begins to wilt, remove from the heat and transfer to serving plates. Sprinkle with the toasted pepitas and the cheese if desired. Serve immediately.