Rinse lettuces, and separate leaves. Let leaves stand in cold water to cover in a large bowl 30 minutes. Drain well, and pat dry with paper towels. Tear leaves into bite-size pieces.
Bake walnuts in a shallow pan at 350° for 10 minutes or until toasted. Stir in butter and 1/2 teaspoon salt.
Pulse shallots in a food processor until minced. Add remaining 1 1/4 teaspoons salt, vinegar, and next 3 ingredients. With processor running, pour oil through food chute. Add 1/4 cup walnuts, and process until smooth.
Toss lettuce and mushrooms with 1/2 cup vinaigrette; top with remaining 1 cup walnuts. Serve with remaining vinaigrette, if desired.