In a small bowl, whisk together the vinegar, shallots, salt, and pepper. Add the olive oil in a thin stream, whisking all the time, until completely emulsified. Set aside.
In a skillet, heat the vegetable oil over medium high heat. Add the onion and cook, stirring frequently, for about 5 minutes, or until golden. Set aside.
In a large serving bowl, combine the spinach leaves with the chiles and the tomatoes. Add the onions and the dressing and toss gently together to coat all the ingredients, but do not break the tomatoes apart.
Serve the salad on individual plates, sprinkling the sesame seeds and cheese over each.
To pickle chipotle peppers, combine 6 peppers that have been stemmed and, if desired, seeded, with 2 cups of red wine vinegar in a small nonreactive saucepan. Bring to a boil, then reduce the heat so the vinegar is simmering. Cook, covered, for 5 minutes. Set aside to cool, then store in a glass jar in the refrigerator for up to 3 months.
c.1997, M.S. Milliken & S. Feniger, all rights reserved