2 dozen cremini or baby bella mushrooms with stems 1 1/2 - 2 inch-wide
2 tablespoons unsalted butter, plus extra for buttering dish
1 large garlic clove - minced
1 1/2 teaspoons fresh oregano - finely chopped
4 oz pecans - finely chopped (1 cup)
1/2 teaspoon salt
1/4 teaspoon black pepper
1 cup heavy cream
Put oven rack in middle position and preheat to 400 degrees.
Trim ends of mushroom stems and carefully separate caps and stems, reserving both. Arrange caps, stemmed sides up, in a buttered 13 by 9-inch shallow baking dish.
Finely chop stems, then cook with garlic and oregano in butter in a 10-inch heavy skillet over moderate heat, stirring frequently, until lightly browned, about 5 minutes. Stir in pecans, 1/4 teaspoon salt, and 1/8 teaspoon pepper and cook, stirring frequently, 1 minute. Stir in 1/3 cup cream and bring to a simmer, then remove from heat.
Sprinkle insides of caps with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper, then divide filling among caps. Drizzle mushrooms with remaining 2/3 cup cream and bake until filling is browned and caps are tender, about 30 minutes. Serve mushrooms drizzled with pan juices.
Cook's note: Stuffed mushrooms can be assembled, but not baked, 1 day ahead and chilled, covered. Bring to room temperature before baking.