Boil the sliced potatoes in water for 20-25 minutes, until knife tender. Preheat the oven to 400 degrees. Grease 9 in x 13 in dish.
While the potatoes boil, start the shepherd's pie filling. Put 1 Tablespoon of olive oil, the sliced carrots, diced onion and minced garlic into a large pan and heat over medium heat. Sauté for 4-5 minutes. Add the ground beef to the pot and mix breaking it up into smaller pieces. Top the beef with 1/2 teaspoon black pepper, 1 teaspoon salt, 1 teaspoon dried thyme, and ½ teaspoon dried rosemary. Mix and cook until the meat is browned, about 5-7 minutes.
Add the peas to the pan and mix, lower heat to low. Add 2 Tablespoons flour to the meat and mix well. Then add the 2 Tablespoons tomato paste, 2 Tablespoons blackstrap molasses, and ½ teaspoon chili powder to the meat and mix. Add 1 cup of strained tomatoes( or marinara), mix, and simmer for 2 minutes. Add 1 cup of chicken broth and simmer until the sauce reduces to a gravy, about 5 minutes.
Pour the shepherd's pie filling into a 9 in x 13 in dish.
Drain the boiled potatoes and put them into a large bowl and top with 1/3 cup milk, 3 Tablespoons butter, 1 teaspoon salt and 1/4 teaspoon pepper. Mash everything together until smooth. Scoop the mashed potatoes on top of the shepherd's pie filling in the pan, evenly distributing it and then smooth it out with a spatula.
Bake for 25-30 minutes.