Preheat the oven to 350°. Spread the walnuts on a baking sheet and toast for about 10 minutes, or until lightly browned and fragrant. Let cool.
Line a 7-by-5-inch loaf pan with plastic wrap, leaving 3 inches of overhang all around. In a large bowl, break up the Roquefort with a fork until slightly chunky. Add the walnuts and crème fraîche and mix thoroughly. Scrape the mixture into the loaf pan and smooth the top. Cover with the overhanging plastic wrap and refrigerate until very firm, at least 6 hours or overnight.
Peel, core and thinly slice the apples. In a medium bowl, toss the apples with 2 tablespoons of the lemon juice. In a small bowl, combine the remaining 2 teaspoons of lemon juice with the olive oil, mustard and balsamic vinegar. Season with salt and pepper. Pour the vinaigrette over the apples and toss well.
Unmold the Roquefort terrine and discard the plastic wrap. Run a thin knife under hot water and wipe it off, then cut the terrine into 3/4-inch-thick slices, rinsing and drying the knife after each cut. Arrange the slices on small plates and serve with the apple salad and bread.
The dominating saltiness of the Roquefort and the unctuousness of the walnuts make an ideal contrast for a Sauternes.