Adjust oven rack to center position and heat oven to 400 degrees. Place butter in a baking pan large enough to hold the pears in a single layer: set in the heating oven until it melts.
Sprinkle sugar over melted butter, then place pears, cut side down, on top. Bake until tender, about 30 minutes.
Remove pan from oven, turn pears over and baste with pan sauce. Return to oven; bake until golden and glossy, about 10 minutes longer.
Let cool slightly. Transfer pears to dessert plates or bowls. Top each with dollop of sour cream, drizzle with caramel pan sauce, sprinkle with nuts and serve.
