How to Make It
Preheat oven to 425°.
Heat a medium saucepan over medium-high heat. Add 1 tablespoon oil to pan; swirl to coat. Add shallots, thyme, and 1/4 teaspoon salt; sauté 4 minutes, stirring frequently. Add sherry; cover, reduce heat, and simmer 10 minutes or until very tender. Stir in vinegar and black pepper. Remove from heat; keep warm.
Heat a large ovenproof skillet over medium-high heat. Add remaining 1 tablespoon oil; swirl to coat. Add mushrooms, fuzzy side down; cook 4 minutes or until browned. Turn mushrooms over; top each with 1 butter piece. Place pan in oven; bake mushrooms at 425° for 5 minutes or until tender. Remove from oven; sprinkle with remaining 1/4 teaspoon salt. Spoon about 1/2 cup shallot mixture onto each of 4 plates; top each serving with 3 mushrooms. Drizzle any pan juices over servings. Sprinkle evenly with chives.