Mixed Greens With Walnuts

Ingredients

• 1 small head Boston lettuce
• 1 small head Bibb lettuce
• 1 small head romaine lettuce
• 1 1/4 cups coarsely chopped walnuts
• 2 tablespoons butter or margarine
• 1 3/4 teaspoons salt, divided
• 2 large shallots, halved
• 2 tablespoons white wine vinegar
• 2 teaspoons sugar
• 1/2 teaspoon ground white pepper
• 1 teaspoon Dijon mustard
• 1 cup olive oil
• 2 (8-ounce) packages sliced fresh mushrooms