In a covered saucepan, cook potatoes in boiling salted water for 12 to 15 minutes or till just tender; drain well. Meanwhile, prepare the Walnut Vinaigrette.
Remove skin and bones from smoked salmon. Coarsely flake; set aside.
In a large mixing bowl combine drained potatoes and sweet pepper strips. Pour half of the Walnut Vinaigrette over the potato mixture. Toss lightly to coat.
To serve, arrange romaine on 4 dinner plates. Top with radicchio and potato mixture on top of the romaine. Sprinkle with walnuts. Arrange salmon on top. Drizzle with remaining Walnut Vinaigrette. Serve immediately. Makes 4 main-dish servings.