Step 1

Preheat oven to 400°F. In a 10-inch cast-iron or oven-going skillet, heat the oil over medium heat. Add the ground beef, carrots, mushrooms, and celery. Cook, stirring occasionally, until vegetables are crisp-tender and beef is heated through, about 4 minutes.

Step 2

Add the tomato paste, mustard, coconut aminos, rosemary, thyme, onion powder, garlic powder, salt, and black pepper; stir to combine.

Step 3

In a 1 cup glass measure or small bowl, whisk together ½ cup of the broth and the arrowroot powder until well combined. Add remaining ½ cup broth. Add broth mixture to the skillet and cook, stirring constantly, until thickened.

Step 4

In a large saucepan or pot, place potatoes, 1 teaspoon of the salt, and water to cover. Bring to a boil over high heat; reduce the heat and simmer, covered, until potatoes are tender, 10 to 15 minutes.

Step 5

Drain the potatoes. Add the oil to the potatoes, and coarsely mash with a potato masher.

Step 6

In a small saucepan, heat the milk and fresh herbs over medium heat just until warm and fragrant. Remove herbs from milk; add to the potatoes along with the sour cream, remaining ½ teaspoon salt, and black pepper. Mash until the potatoes are smooth.

Step 7

Spoon the mashed potatoes onto the hot filling. Bake until the filling is bubbly and mashed potatoes are heated through, 10 to 15 minutes. Sprinkle with chives.