Pour 1 and 1/2 tablespoons olive oil in a large (oven-safe to save a dish!) skillet over medium high heat. Add garlic cloves and finely chopped onion and cook, stirring frequently for about 1 minute. Add the finely chopped carrots and celery and cook for another 2-3 minutes or until veggies are softened.
Increase heat to high and add in the beef. Cook, breaking up the beef, until browned through. If there is excessive grease, drain off that additional fat. Add in the flour and cook an additional minute to cook off the flour taste. Reduce heat to medium-high and add in the frozen peas or corn, can of diced tomatoes, tomato paste, beef stock, grape juice, beef bouillon cube, Worcestershire sauce, dried thyme, and 2 bay leaves.
Bring this mixture to a simmer then reduce heat to medium and bring the mixture to a rapid simmer. Cook for 25-35 minutes at medium heat, stirring every 5 or so minutes, until you have a good thickened gravy. Remove from heat, cover, and refrigerate until ready to bake. (I like making in the morning and assembling before dinner). If making immediately, place in the fridge until potatoes are done (you want the mixture to chill for as long as possible so the layers stay separate when baking, 15-20 minutes at a minimum) :) If you didn't prepare in an oven-safe skillet, transfer to a 2 quart oven-safe dish.
Preheat the oven to 350 degrees F. Peel and then cube the potatoes into golf-ball sized pieces. Place in a strainer and rinse under cold water until the water rinsing through comes out clear. Place in a large pot and cover with cold water. Add 1 tablespoon fine sea salt and 1 bay leaf, stir, and bring to a boil. Reduce to a simmer until the potatoes are fork-tender; about 15-20 minutes. When a potato can easily be pierced, drain the potatoes in a strainer and make sure you get rid of ALL the extra moisture. Let them dry for about 3 minutes. Discard the bay leaf.
While the potatoes are drying in the strainer, add the heavy cream, sour cream, and butter to the pot. Melt on low heat and stir to combine. While potatoes are still hot, put them through a ricer and add right on top of the butter/sour cream mixture. Stir with a wooden spoon until smooth. Season to taste with salt and pepper (I use about 1/2 teaspoon salt and 1/4 teaspoon pepper). Cover and place in the fridge until ready to assemble pie! OR add these mashed potatoes to top the pie in an even layer.
Use a fork to add marks into the surface of the potatoes. Drizzle with remaining 1/2 tablespoon olive oil.
Bake in preheated oven for 25-30 minutes or until the potatoes are golden brown and the edges are bubbling. Broil for 1-2 minutes on high to get the very tops of the potatoes crispy! Remove from heat and garnish with fresh thyme or fresh parsley as desired. Serve hot :)