Yields: 6-8 servings
Heat olive oil in a large skillet over medium heat. Add garlic cloves and finely chopped onion and cook, stirring frequently for about 1 minute. Add the finely chopped celery; cook for another 2-3 minutes or until veggies are softened.
Increase heat to high and add in the beef. Cook beef, breaking it up, until browned through. If there is excessive grease, drain. Add in the flour and cook an additional minute. Reduce heat to medium high and add in the frozen veggies, roma tomatoes, tomato paste, beef stock, beef base, Worcestershire sauce, and dried thyme.
Bring mixture to a simmer then reduce heat to medium. Cook for about 25-35 minutes, stirring occasionally, until gravy has thickened. Remove from heat, cover, and refrigerate until ready to bake. If making immediately, place in the fridge until potatoes are done to chill as long as possible (at least 20 minutes). If needed, transfer to a 2-quart oven-safe dish.
Preheat oven to 350 degrees F. Peel potatoes and cut into 1/2 inch dice. Place in a large pot with cold, lightly salted water; cover and set on high heat. Once boiling, uncover and reduce heat to a simmer until the potatoes are fork-tender; about 15-20 minutes. Drain the potatoes in a colander and return to pan.
Add the heavy cream, sour cream, and butter to the pot. Melt on low heat and stir to combine. Mash the potatoes and then add to the cream mixture. Stir with a wooden spoon until smooth. Season to taste with salt and pepper. Cover and place in the fridge until ready to assemble pie, add mashed potatoes to top the pie in an even layer.
Using a fork, add marks into the surface of the potatoes. Drizzle with remaining 1/2 tbsp of olive oil.
Bake in preheated oven for 25-30 minutes or until the potatoes are golden brown and the edges are bubbling.
Remove from oven. Serve with a garnish of fresh thyme or parsley as desired.