1. For the topping: Put the potatoes in a large pot with enough cold water to cover by about 1 inch, add 1 teaspoon salt. Bring to a boil over high heat. Reduce the heat to medium-high and cook until the potatoes are very tender, about 15 minutes
2. For the filling: Meanwhile, preheat the oven to 375 degrees F. Heat 2 tablespoons butter in a large skillet over medium-high heat. Add the onion and cook, stirring often ,until soft, 3 to 5 minutes. Stir in the flour and cook 1 minute longer. Add the tomatoes and broth. Cook, stirring often, until reduced and thickened, 7 to 10 minutes.
3. Remove the meat from the chicken, discarding all skin and bones. Cut or shred the meat into small pieces. (You should have about 2 cups.)
4. Stir the chicken and spinach into the sauce and season with salt and pepper. Transfer the filling to a 2-quart casserole dish or 4 to 6 individual oven safe ramekins.
5. Drain the potatoes well and force through a ricer or food mill. (You should have about 2 cups). Add the remaining 4 tablespoon butter, the cheese, and season with salt and pepper. Stir in the egg yolk. Spread the potatoes over the chicken or transfer to a large pastry bag fitted with a wide tip and pipe evenly over the chicken. Bake until the topping is lightly browned, and the chicken is heated through, about 20 to 30 minutes.
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