2 lb. Yukon Gold potatoes, scrubbed, cut into 2" pieces
1 tsp. kosher salt, plus more
4 garlic cloves, finely chopped
½ cup whole milk
4 Tbsp. unsalted butter
¼ tsp. coarsely ground black pepper
Filling and assembly:
1 lb. ground chicken
3 scallions, thinly sliced
Zest of 1 lime
½ habanero chile, seeds removed, finely chopped
1 Tbsp. Angostura bitters
1½ tsp. kosher salt
½ tsp. coarsely ground black pepper
2 Tbsp. vegetable oil
1 medium sweet onion (such as Vidalia, Walla Walla, or Maui), finely chopped
8 large garlic cloves, finely chopped
1 medium tomato, coarsely chopped
2 medium carrots, peeled, finely chopped
1 1" piece ginger, peeled, finely chopped
¼ cup ketchup
1 Tbsp. chopped thyme
2 oz. white cheddar cheese, coarsely grated (about ½ cup)
Place potatoes in a large pot and pour in water to cover by 1"; season generously with salt. Bring to a boil, reduce heat, and simmer until fork-tender, 10–12 minutes.
Drain potatoes and return to pot. Add garlic, milk, butter, pepper, and 1 tsp. salt. Using a potato masher or pastry cutter, mash potatoes until smooth. Cover and set aside.
Filling and assembly:
While potatoes are cooking, mix chicken, scallions, lime zest, habanero, bitters, salt, and pepper in a medium bowl with a fork to combine. Let sit 10 minutes.
Preheat oven to 375°. Heat oil in a large skillet with a lid over medium. Add chicken mixture and cook, breaking up with a wooden spoon and stirring often, until no longer pink, about 4 minutes. Add onion and garlic and cook, stirring occasionally, until softened and translucent, about 5 minutes. Stir in tomato, carrots, ginger, ketchup, thyme, and 1 cup water. Bring to a simmer, then cover and cook, stirring occasionally and adjusting heat as needed to maintain a simmer, until filling is thickened slightly and carrots are softened, 10–12 minutes.
Transfer filling to a 3-qt. baking dish and spread into an even layer. Spoon reserved mashed potatoes over and smooth into an even layer; sprinkle cheese on top. Bake shepherd’s pie until filling is bubbling around the edges and cheese is melted, about 20 minutes.
Heat broiler. Broil pie until cheese is beginning to brown, about 4 minutes. Let sit 10 minutes before serving.