1. Combine first 4 ingredients in a small heavy saucepan over medium-high heat; bring to a boil. Cook until reduced to about 1/4 cup (about 8 minutes). Remove from heat; discard thyme sprigs. Add butter, 1 piece at a time, stirring constantly with a whisk until butter is thoroughly incorporated. Strain mixture through a fine sieve over a bowl, pressing to release all of the sauce; discard solids. Stir in sugar, 1/4 teaspoon salt, and 1/8 teaspoon pepper.
2. Heat a large cast-iron skillet over high heat. Coat pan with cooking spray. Sprinkle both sides of scallops evenly with remaining 1/4 teaspoon salt and remaining 1/8 teaspoon pepper. Add scallops to pan; cook 2 minutes. Turn scallops over; cook 1 minute or until desired degree of doneness. Serve scallops with sauce; sprinkle with chopped thyme, if desired.