Preheat the oven to 350 degrees F. Spread the walnuts on a baking sheet. Bake until toasted, 6 to 7 minutes. Let cool.
Make the pickled shallot: Combine the vinegar, honey and 3/4 teaspoon salt in a small microwave-safe bowl. Microwave until hot, about 30 seconds. Add the shallot and let sit 15 to 20 minutes.
Combine the lettuces and radicchio in a large bowl. Remove the shallot from the pickling liquid and add to the salad along with the toasted walnuts. Whisk the sour cream, olive oil and mustard into the pickling liquid; season with pepper. Drizzle over the salad, season with salt and pepper and toss.