Place cream cheese and 2 tablespoons butter in bowl of electric stand mixer. Attach flat beater to mixer. Turn to medium and beat about 1 minute. Stop and scrape bowl. Add 3/4 cup flour. Turn to low and mix about 1 minute, or until well blended. Form mixture into ball. Wrap in waxed paper and chill 1 hour. Clean mixer bowl and beater. Divide chilled dough into 24 pieces. Press each piece into miniature muffin cup (greased, if desired). Meanwhile, melt remaining 1 tablespoon butter in 10-inch skillet over medium heat. Add mushrooms and onions. Cook and stir until tender. Remove from heat. Cool slightly. Place egg, remaining 1 teaspoon flour, and thyme in mixer bowl. Turn to medium-high and beat about 30 seconds. Stir in cheese and cooled mushroom mixture. Spoon into pastry-lined muffin cups. Bake at 375° for 15 to 20 minutes, or until egg mixture is puffed and golden brown. Serve warm.