Heat 2 teaspoons oil in a large skillet over medium-high heat. Add mushrooms; cook, stirring occasionally, until they begin to give off liquid, about 3 minutes. Stir in leeks; cook, adjusting heat to prevent scorching, until the vegetables are very tender and the leeks are starting to brown, about 8 minutes. Add brandy (or vermouth) and broth; cook until reduced and thickened, about 30 seconds. Remove from the heat and stir in sour cream, parsley, 1/4 teaspoon salt and pepper to taste. Cover and set aside.

Pat scallops dry and sprinkle with the remaining 1/4 teaspoon salt and pepper. Heat the remaining 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add the scallops and cook until golden brown, 2 to 3 minutes per side. To serve, place some of the leek-mushroom mixture on each plate and top with scallops.

Note: Be sure to buy "dry" sea scallops (scallops that have not been treated with sodium tripolyphosphate, or STP). Scallops that have been treated with STP ("wet" scallops) have been subjected to a chemical bath and are not only mushy and less flavorful, but will not brown properly.