Thinly slice 2 medium shallots into a medium heatproof bowl.
Combine 1/3 cup white balsamic vinegar, 1 1/2 teaspoons kosher salt, and 1 teaspoon sugar in a small saucepan and bring to a boil, stirring until the sugar dissolves. Pour the boiling liquid over the shallots and let sit at room temperature until cool.
Drain the shallots into a strainer set over a medium bowl; set the shallots aside.
Add 2 teaspoons Dijon mustard and a little freshly ground black pepper to the pickling liquid and stir to combine. While whisking, slowly add 2 tablespoons olive oil until the dressing is smooth and combined.
Cut 1 orange into segments by slicing off the top and bottom. Stand the orange on a cut end and use a paring knife to remove the peel and white pith, cutting downward along the curve of the fruit. Slice between the membranes to release the orange segments. Slice each segment in half, or thirds if very large.
Squeeze the juice from the orange membrane over the bowl of dressing and stir to combine.
Combine 1 pound mixed greens, washed and thoroughly dried, 1/2 cup pomegranate seeds, the orange segments, and the pickled shallots in a large salad bowl. Drizzle about one-third of the dressing over the top and toss to combine. Taste, then add more dressing, kosher salt, and freshly ground black pepper as needed.