Make the dressing:
In a medium bowl, whisk together the vinegar, mustard, and garlic. Slowly whisk in the olive oil until the mixture is smooth and emulsified. Add the herbs, season with salt and pepper, and mix well. Set aside.
Make the salad:
Tip:Dress warm salads just before serving— they don’t take well to sitting around.
Heat the oven to 350°F. Put the spinach in a large mixing bowl and set aside. Season the tenderloin with salt and pepper. In a large, ovenproof sauté pan, heat the olive oil over high until very hot, add the tenderloin, and sear well, about 2 minutes on all sides. Push the tenderloin aside and add the shallots to the pan, shaking the pan to coat the shallots with the pan drippings. Put the pan in the oven and roast the tenderloin and shallots until the tenderloin is medium rare (130°F), 15 to 18 minutes, or to desired doneness (the USDA recommends 160°F, but I find that it makes the pork much too dry). Remove the pork from the pan and let it rest on a cutting board for 5 to 10 minutes. Slice it thinly on the bias.
Set the sauté pan over high heat, add the dressing, and bring it just to a boil, scraping the shallots and any pan drippings into the dressing. Pour the dressing over the spinach, add the cranberries and pear, and toss well. Use tongs to distribute the greens, cranberries, and pear slices evenly on four plates. Top with the pork slices and toasted walnuts. Garnish with red onion slices, if you like.