Instructions: Halve portobellos and cut into 1/4-inch thick slices. Cut cremini into 1/4-inch thick slices. Chop shallots and in a 12-inch heavy skillet cook half of shallots in half of butter over moderate heat, stirring, 1 minute. Add portobellos and salt and pepper to taste and cook, stirring occasionally, until liquid mushrooms give off is evaporated, about 15 minutes. Transfer portobellos to a bowl and keep warm, covered.

Cook remaining shallots in remaining butter over moderate heat, stirring 1 minute. Add cremini and salt and pepper to taste and cook, stirring occasionally, until liquid mushrooms give off is evaporated, about 10 minutes. Add portobellos to cremini and keep warm, covered. Mushrooms may be prepared up to this point 1 day ahead and chilled, covered.

Diagonally cut scallions into thin slices. In a bowl whisk together vinegar, soy sauce, and sugar. Heat mushrooms over moderately-high heat until hot and add vinegar mixture. Boil mixture 3 minutes, or until liquid is reduced slightly, and stir in scallions.

This recipe yields 4 servings.

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