Shepherd's Pie with Chicken and Spinach

Ingredients:

Topping

• 2 Yukon Gold potatoes, peeled and cut into large pieces (about 340 g)

Filling

• 6 tablespoons unsalted butter
• 1 medium onion, chopped
• 2 tbsp. premium flour
• 3 large ripe tomatoes, cored and diced
• 1 cup lightly salted chicken broth
• 1 grilled chicken (about 1 kg)
• 1 package (280g) frozen chopped spinach, thawed and squeezed out of excess water
• 1 cup grated cheddar
• 1 large egg yolk

Preparation:

1. Topping:
   
   Place the potatoes in a large saucepan and add enough cold water to cover by about 2 cm. Add 1 teaspoon of salt. Bring to a boil over high heat. Reduce the heat to medium and simmer until the potatoes are very tender, about 15 minutes.
   
2. Meanwhile, preheat oven to 190°C.
3. Filling:
   
   In a large skillet, heat 2 tablespoons of butter over medium heat. Add the onion and cook, stirring frequently, until softened, 3-5 minutes. Add the flour and cook for another minute. Add the tomatoes and broth. Cook, stirring frequently, until the liquid has reduced slightly and thickened, 7-10 minutes.
4. Remove the chicken, discarding any skin and bones. Chop the meat or shred it with two forks. You should have about 2 cups of chicken. Stir the chicken and spinach into the sauce, season with salt and pepper. Transfer the filling to a 2-quart baking dish or 4-6 individual ramekins.
5. Thoroughly drain the potatoes and puree them using a vegetable mill. You should have about 2 cups of mashed potatoes. Add the remaining 4 tablespoons of butter and the cheese, season with salt and pepper to taste. Add the egg yolk. Arrange the potatoes on top of the chicken and spread them out, or use a large pastry bag fitted with a wide tip and pipe evenly onto the chicken. Bake until the topping is lightly browned and the chicken is heated through, about 20-30 minutes.