Make the mashed potatoes: Fill a large pot with water and bring to a boil. Add the cubed potatoes and simmer on medium-high heat, until fork tender, 15 minutes. Strain the potatoes and add back to the pot. Add the butter, milk, garlic powder, salt, and pepper. Mash everything together with a potato masher (or a fork) until smooth and combined. Cover the pot with a tea towel and secure with the pot lid. Place the mashed potatoes to the side until ready to use.

Make the lentil filling: Preheat the oven to 375°F. In a large deep skillet, add the olive oil and bring to medium heat. Add the onion, garlic, carrots, and celery. Cook the vegetables, stirring frequently, until softened, 10 minutes. Add the mushrooms and increase heat to medium-high. Cook until the mushrooms are softened and browned, 10 minutes. Reduce heat to medium, sprinkle in the flour, and stir to combine.

Pour in the vegetable broth and bring to a low simmer. Add the balsamic vinegar, fresh thyme, tomato paste, smoked paprika, fennel seeds, and salt. Stir together. Toss in the cooked lentils and peas, cook until the peas are vivid green, 1 minute.

Assemble the shepherd’s pie: Pour the filling into a medium casserole dish (8½ x 11) and spoon the mashed potatoes on top. Smooth with the back of a spoon to make an even layer. If desired, lightly graze the mashed potatoes with a fork to make fork lines through the top. Season with salt and pepper.

Bake for 20 minutes, or until the mashed potatoes turn lightly golden and the lentil filling bubbles around the edges. Increase heat to broil for 3-5 minutes to lightly crisp the mashed potato topping.

Remove from the oven and let cool for 10 minutes. If desired, sprinkle with fresh thyme.