Put oven rack in middle position in oven and preheat to 400°F.
Trim ends of mushroom stems and separate caps and stems, reserving both. Arrange caps, stemmed sides up, in a buttered 13"x9" shallow baking dish.
Finely chop stems(or process in food processor), then cook with garlic and oregano in butter(or olive oil) in a 10" heavy skillet over medium heat, stirring frequently, until lightly browned, about 5 minutes. Stir in pecans, bread crumbs, 1/4 teaspoon salt, and 1/8 teaspoon pepper and cook, stirring frequently, 1 minute. Stir in 1/3 cup half and half 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 the caps.
Drizzle mushrooms with the 2/3 cup cream and bake until filling is browned and caps are tender, about 30 minutes. Sprinkle over parsley to garnish. Serve mushrooms drizzled with pan juices.
To make ahead:
Stuffed mushrooms can be put together, but not baked, 1 day ahead and chilled, covered. Bring to room temperature before baking.