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.
In a large heavy skillet over medium heat, melt 4 tablespoons butter, and cook the chopped mushrooms and scallions, stirring occasionally, until liquid has been released and then skillet becomes almost dry, about 20 minutes. Stir in thyme, salt, and pepper; cook 2 more minutes. Add sherry, and cook until skillet is almost dry, 4 minutes. Stir in lemon juice. Remove from heat, and let cool.
In a large bowl, combine the mushroom mixture with the reserved toasted walnuts, parsley, Tabasco sauce, and cream cheese. Season with salt and pepper to taste.
Line a 12 1/4-by-4-by-3-inch terrine (or use any 6-cup mold) with plastic wrap, allowing for a 4-inch overhang on all sides. Spoon the mushroom mixture into the terrine. Firmly press down all over terrine with your hands, spreading mixture as evenly as possible. Cover the mold with the overhanging plastic wrap, and chill in the refrigerator for at least 8 hours or overnight.
Just before serving, melt the remaining 2 tablespoons butter in a medium skillet, and cook the halved mushrooms just until tender, about 5 minutes. Season mushrooms with salt and pepper. Unwrap the chilled terrine, and invert it onto a large serving platter. Use a warm, wet towel on top of the mold, if necessary, to help loosen the terrine. Garnish the serving platter with the cooked mushroom halves, remaining walnuts, and thyme leaves. Serve with toast points or crackers.