Bitter Greens with Carrots, Turnips, and Oranges

1/2 cup walnuts 2 tablespoons Champagne vinegar or Riesling vinegar 2 tablespoons fresh lemon juice 1 tablespoon finely chopped shallot 1 tablespoons whole grain mustard 1/3 cup grapeseed oil or olive oil Kosher salt, freshly ground pepper 1 teaspoons thyme leaves 3 cups watercress, tough stems removed 1/2 head of radicchio, halved lengthwise, core removed, leaves separated 6 baby carrots, scrubbed, thinly sliced into rounds 4 baby turnips, preferably Tokyo, tops trimmed to 1/2", scrubbed, very thinly sliced lengthwise 2 oranges, peel and white pith removed, sliced into rounds, seeds removed 1/4 cup kumquats, sliced into rounds, seeds removed (optional) Flaky sea salt (for serving)