Step 1: Preheat your oven to 350°F (175°C). Grease a loaf pan (8.5 x 4.5 inches or similar terrine mold) with cooking spray or butter and line it with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a skillet, heat the olive oil over medium heat. Add the chopped leeks and sauté for about 5-7 minutes until they are soft and translucent. Remove from heat and let cool completely.
Step 3: In a large mixing bowl, combine the ground chicken, cooled sautéed leeks, chopped apricots, heavy cream, eggs, fresh thyme, salt, black pepper, and ground nutmeg. Mix until all ingredients are well combined, but do not overmix to keep the chicken tender.
Step 4: Pour the chicken mixture into the prepared loaf pan, pressing it down gently with a spatula to eliminate any air pockets and create an even top surface.
Step 5: Place the loaf pan into a larger baking dish. Fill the larger baking dish with hot water until it reaches about halfway up the sides of the loaf pan (this creates a water bath).
Step 6: Bake in the preheated oven for about 1 hour to 1 hour and 15 minutes, or until the internal temperature of the terrine reaches 165°F (75°C) when measured with a meat thermometer, and it feels firm to the touch.
Step 7: Once cooked, carefully remove the terrine from the oven and the water bath. Let it cool in the pan for at least 30 minutes at room temperature. Then, refrigerate for at least 4 hours, or preferably overnight, to allow it to fully set and for the flavors to develop.
Step 8: When ready to serve, use the parchment paper overhang to carefully lift the terrine out of the pan. Slice it into thick, even pieces. Garnish with fresh chopped parsley and serve cold or at room temperature.