Reduce heat to medium and add 4 tbsp. butter to the skillet. Once melted, stir in sliced fennel, 1 tbsp. thyme, 1 tsp. kosher salt, and 1/2 tsp. pepper. Cook, stirring frequently, until the fennel is very soft and golden brown, about 15 minutes. Remove skillet from heat; stir in the brandy and 1/4 cup of water. Return skillet to the heat and cook, scraping the bottom of the pan to release any brown bits, until the liquid evaporates, 3 to 4 minutes.