The pate, which should be chilled for at least eight hours, may be made up to one day ahead. Serve with toast points made from thin-sliced white sandwich bread; remove crusts, and toast just until golden.
Heat oven to 350 degrees. Spread walnuts on a baking pan. Bake until fragrant, about 7 minutes. Transfer walnuts to a bowl; set aside to cool. Finely chop, and reserve.
In a large heavy skillet over medium heat, melt 4 tablespoons butter. Add mushrooms, scallions, and thyme, and cook, stirring occasionally, until liquid has been released and skillet becomes almost dry, 5 to 8 minutes. Stir in salt, pepper, and sherry. Cook until skillet is almost dry, about 10 minutes more. Remove from heat, and let cool.
In a large bowl, combine mushroom mixture with reserved walnuts, parsley, lemon juice, Tabasco, and cream cheese. Season with salt and pepper to taste.
Line a 3-cup mold with plastic wrap, allowing for a 4-inch overhang on all sides. Spoon the mushroom mixture into the pan. Cover with overhanging plastic, and firmly press down all over terrine with hands; spread mixture as evenly as possible. Refrigerate for at least 8 hours, or overnight.
Just before serving, melt remaining 2 tablespoons butter in a medium skillet, and cook halved mushrooms just until tender, about 5 minutes. Season with salt and pepper. Unwrap chilled terrine, and invert it onto a small serving platter. Surround with cooked mushroom halves, and thyme leaves. Serve with toast points, water biscuits, or any plain cracker.
Serves 1 to 2 dozen. 1 cup walnuts; 2 1/2 pounds assorted mushrooms; 1 1/2 tablespoons fresh thyme leaves; juice of 1 lemon