7 ounces walnuts
The roasted walnuts and vinaigrette can be made 1 day ahead. Store the walnuts airtight at room temperature. Cover and refrigerate the vinaigrette. Let the vinaigrette stand at room temperature 15 minutes and rewhisk before serving.
Preheat the oven to 350°F.
Place the walnuts on a baking sheet and roast for 3 to 5 minutes, or until hot. Remove from the oven, dust with powdered sugar, and return the baking sheet to the oven for 3 to 5 minutes, or until toasted and fragrant. Set the nuts aside to cool completely.
In a large bowl, whisk 1.75 ounces of the goat cheese and 3 1/2 ounces of the vinegar together to blend. Slowly add the oil while whisking. Depending on the strength of the goat cheese and oil, you may need to whisk in a little water to lighten up the dressing. Mix in the tarragon. Season the vinaigrette to taste with salt and pepper.
In a large bowl, toss the arugula, spinach, and frisée with enough goat cheese dressing to coat. Add the remaining goat cheese and three-fourths of the walnuts and gently toss to combine. Season to taste with salt and pepper.
Divide the salad among 4 plates and garnish with the remaining walnuts.