Step 1

Boil potatoes: Add potatoes to a large pot of water. Cover with a lid and bring to a boil. Boil until fork-tender, 15 to 20 minutes. Drain well (let the potatoes sit in the colander for 1 minute after draining, to release excess moisture).

Step 2

Mash potatoes: While the potatoes are still warm, run them through a potato ricer over the pot (or alternatively use a potato masher). Add 6 tablespoons of the milk, 5 tablespoons of the melted butter, garlic powder, salt and ¼ cup of the Parmesan. Stir with a rubber spatula to mix. Taste and add the remaining 2 tablespoons of milk and 1 tablespoon of the butter if needed. You want the mashed potatoes to be on the looser side (not too stiff). Stir until smooth. Cover and keep warm on lowest heat.

Step 3

Preheat: Put a rack in the upper-third of the oven and preheat to 400°F (200°C).

Step 4

Start filling: Heat the olive oil in a large nonstick skillet over medium-high heat. Sauté the onion until translucent, about 5 minutes. Add the meatless ground. Use a spatula to break up the ground into smaller pieces, and sauté until crisp on the edges, 8 to 12 minutes. Add tomato paste, stir well to mix. Add carrots and celery, sauté until they begin to soften, about 3 minutes.

Step 5

Finish filling: Reduce the heat to medium. Add garlic, thyme, rosemary, salt and pepper. Cook stirring frequently, until fragrant, about 2 minutes. Add flour, then toss to coat vegetables evenly. Add stock and stir well. Once the mixture is bubbling, stir in frozen corn and peas. Remove from heat. Stir in parsley.

Step 6

Bake: Transfer filling to 1 (2-quart) baking dish (see note 5). Top with mashed potatoes, using the back of a spoon to make swoops in the surface (this way, the ridges will get golden brown in the oven and melted butter will nestle in the pockets). Bake for 30 to 35 minutes, or until the mashed potato ridges are golden brown.

Step 7

Garnish and bake: Drizzle the remaining 1 tablespoon of melted butter on top, and sprinkle with remaining ¼ cup Parmesan. Return to the oven until cheese is melted, about 5 minutes. Garnish with more parsley and cheese.