Preheat your oven to 375°F (190°C).
In a large pot, boil the chopped potatoes until they are fork-tender. Drain the water and return the potatoes to the pot.
Mash the potatoes using a potato masher or a fork. Add the plant-based milk, vegan butter or olive oil, salt, and pepper. Mix well until you achieve a smooth and creamy consistency. Set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced fennel, celery, and onion. Sauté for about 5 minutes until they start to soften.
Add the peas, diced carrots, and vegan ground meat substitute to the skillet. Cook for another 5 minutes, stirring occasionally.
Stir in the tomato paste, marinara sauce, and your choice of seasonings. Mix well to combine all the flavors. Cook for an additional 2-3 minutes.
Transfer the filling mixture to a baking dish and spread it evenly.
Spoon the mashed potato topping over the filling, spreading it out to cover the entire surface.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and the filling is bubbling.
Remove from the oven and let it cool for a few minutes before serving.
Enjoy your delicious vegan Shepherd's Pie! Feel free to customize the recipe by adding or substituting vegetables and seasonings to suit your taste.