Preheat your oven to 400°F (200°C).

Place the peeled and quartered Russet potatoes in a large pot, cover with cold water by about an inch, add a generous pinch of salt, and bring to a boil. Cook for 15-20 minutes, or until fork-tender. Drain well and set aside.

While potatoes cook, heat olive oil in a large oven-safe skillet or Dutch oven over medium-high heat. Add the chopped mushrooms and cook for 4-5 minutes, stirring occasionally, until deeply browned. Season with a pinch of salt and pepper.

Add 2 tablespoons of unsalted butter to the skillet with the mushrooms. Once melted, add the diced yellow onion, carrots, and celery. Cook for 5-7 minutes, stirring until the vegetables soften.

Stir in the minced garlic, dried marjoram, and rubbed sage. Cook for 1 minute until fragrant.

Add the ground turkey to the skillet, breaking it up with a spoon as it cooks. Continue to cook until no pink remains, about 8-10 minutes. Drain any excess fat if necessary.

Sprinkle the all-purpose flour over the turkey and vegetables, stirring well to combine. Cook for 1-2 minutes.

Stir in the tomato paste, Dijon mustard, and Worcestershire sauce. Mix thoroughly.

Pour in the chicken broth and bring to a simmer, stirring until the sauce thickens. Season the filling with 1/2 teaspoon salt and 1/4 teaspoon black pepper, or to taste. Stir in the frozen peas until just heated through. Remove the skillet from heat.

Mash the drained potatoes with 1/3 cup milk, 3 tablespoons unsalted butter, 1/2 teaspoon salt, and 1/4 teaspoon black pepper until smooth and creamy. If using, stir in the optional minced chives.

Spoon the mashed potatoes evenly over the turkey filling in the skillet (or transfer the filling to a 9x13 inch baking dish if your skillet isn't oven-safe). Spread to the edges and sprinkle with 1/3 cup freshly grated Parmesan cheese.

Bake for 20-25 minutes, or until the topping is golden brown and the filling is hot and bubbly. Let stand for 5-10 minutes before serving.