VEGGIES: Heat 1 and 1/2 tablespoons of olive oil in a large oven-safe skillet over medium-high heat. Add garlic and finely chopped onion, stirring frequently for about 1 minute. Then, add finely chopped carrots and celery, cooking for another 2-3 minutes until veggies are softened.
BEEF: Heat skillet to high and add beef. Cook until browned, breaking it apart. If there's excess grease, drain it. Stir in flour and cook for a minute to remove raw flour taste. Reduce heat to medium-high and add the frozen peas or corn, diced tomatoes, tomato paste, beef stock, grape juice, beef bouillon cube, Worcestershire sauce, dried thyme, and bay leaves.
COOK: Bring mixture to a simmer, then reduce heat to medium and let it rapidly simmer. Cook for 25-35 minutes at medium heat, stirring every 5 minutes until gravy thickens. Remove from heat, cover, and refrigerate until ready to bake. If making immediately, refrigerate until potatoes are ready. To maintain separate layers while baking, chill the mixture for at least 15-20 minutes. If you didn't prepare in an oven-safe skillet, transfer to a 2 quart oven-safe dish.
POTATOES: Preheat the oven to 350°F. Peel and cube potatoes into golf-ball sized pieces. Rinse them under cold water in a strainer until the water runs clear. Transfer to a large pot, cover with cold water, add 1 tablespoon of fine sea salt and 1 bay leaf, then bring to a boil. Reduce to a simmer and cook until the potatoes are fork-tender, about 15-20 minutes. Drain potatoes in a strainer, ensuring all excess moisture is removed. Let them dry for about 3 minutes and discard the bay leaf.
POTATOES CONT.: As potatoes dry in strainer, melt heavy cream, sour cream, and butter in the pot on low heat, stirring to combine. While the potatoes are still hot, put them through a ricer and add right on top of butter/sour cream mixture. Stir until smooth with a wooden spoon. Season to taste with salt and pepper (about 1/2 teaspoon salt and 1/4 teaspoon pepper). Cover and refrigerate until ready to assemble the pie, or use immediately to top the pie evenly.
ASSEMBLY: Use a fork to add marks into the surface of potatoes. Drizzle with remaining 1/2 tablespoon olive oil.
BAKE: Bake in the preheated oven for 25-30 minutes, or until potatoes are golden brown and edges are bubbling. For crispy tops, broil on high for 1-2 minutes. Remove from heat and garnish with fresh thyme or parsley as desired. Serve hot.
Video
Notes
Note 1: Traditional Shepherd's Pie typically includes red wine in the gravy. To do so, substitute 1/2 cup of medium-bodied red wine (like pinot noir) for an equal amount of vegetable broth.