Instructions: Melt 2 tablespoons butter in heavy large skillet over medium-low heat. Add leek and garlic and saute until almost tender, about 5 minutes. Increase heat to medium-high. Add mushrooms and saute until golden brown, about 6 minutes. Add Marsala and stock and boil until liquid is reduced by half, about 4 minutes. Strain sauce, reserving mushrooms. (Can be prepared 1 day ahead. Cover strained sauce and mushrooms separately and chill.)

Prepare barbecue (medium-high heat) or preheat broiler. Brush steaks with olive oil. Sprinkle with salt and pepper. Grill to desired doneness, about 4 minutes per side for medium-rare. Transfer to platter and tent with foil.

Bring sauce to simmer skillet. Remove from heat. Gradually whisk in remaining 2 tablespoons butter. Add reserved mushrooms and stir over low heat until mushrooms are heated through, about 2 minutes. Season to taste with salt and pepper.

Place steaks on plates. Spoon sauce and mushrooms over and serve.

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