Preheat oven to 375 degree F. Halve tomatoes lengthwise. Scoop out seeds and center membrane. Place tomatoes in a shallow roasting pan, cut side up; drizzle with olive oil. Combine thyme leaves, salt, and pepper; sprinkle mixture over tomatoes.
Roast, uncovered, for 45 minutes. Remove from oven. Sprinkle lightly with sugar, and top with thyme sprigs. Serve warm. Makes 4 to 6 servings.