POTATOES: 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 and cook 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.
VEGGIES: While potatoes are cooking, pour 1 and 1/2 tablespoons olive oil in a large oven-safe skillet (1.5 quart skillet) over medium high heat. Add finely chopped onion and cook, stirring frequently for about 1 minute. Add in the garlic and stir for 30 seconds. Add the finely chopped carrots and celery and cook for another 3-5 minutes or until veggies are softened, stirring occasionally. Meanwhile, thinly slice the mushrooms. Add in the mushrooms and cook another 3-5 minutes, stirring occasionally until mushrooms are tender. Add in the flour and stir for 1 minute to cook off the rawness of the flour. Add in the tomato paste, thyme, Italian seasoning, paprika, and salt and pepper to taste. (I add 1 teaspoon salt, 1/2 teaspoon pepper, add to preference). Stir for another 1-2 minutes. Add in 1/2 cup of the vegetable broth (see note 2) and stir for 1-2 more minutes or until thick.
FINISHING VEGGIE BASE: Reduce heat to medium and add in the can of diced tomatoes, frozen peas, frozen corn, and remaining 1 bay leaf. Add in the remaining 1 cup of vegetable stock and stir until combined. Bring this mixture to a simmer then reduce heat to medium and cook for 25-35 minutes, stirring every 5 or so minutes, until you have a thickened gravy. Stir in the red wine vinegar. Preheat the oven to 350 degrees F.
FINISHING VEGGIE BASE CONT.: Remove veggies from heat, cover, and refrigerate until ready to bake. (I like making this in the morning and assembling right before dinner). If making immediately, place in the fridge until potatoes are completely finished (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 1.5-2 quart oven-safe dish. Remove the bay leaf and discard.
POTATOES CONT.: While the potatoes are drying in the strainer, add the heavy cream, sour cream, and butter to the pot the potatoes cooked in. Melt on low heat and stir to combine. While potatoes are still hot, put them through a ricer and add 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.
ASSEMBLY: Gently spread the potatoes in an even layer on top of the veggies. Use a large spoon or silicone spatula to press them to cover all the veggies and make sure to "seal" the edges with potatoes. Use a fork to add marks onto the surface of the potatoes. Drizzle with the remaining 1/2 tablespoon olive oil.
BAKE: 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! (See note 2) Remove from heat and garnish with fresh thyme or fresh parsley as desired. Serve hot.