Heat 1 and 1/2 tablespoons olive oil in a large, oven-safe pan over medium-high heat. Add garlic and finely chopped onion, stirring frequently, for about 1 minute. Add finely chopped carrots and celery, cooking another 2–3 minutes until veggies are softened.
Heat pan to high and add beef. Cook until browned, breaking it apart. Drain any excess grease. Stir in flour and cook for 1 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.
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 pan, transfer to a 2-quart oven-safe dish.
Preheat oven to 350°F. Peel and cube potatoes into golf-ball sized pieces. Rinse them under cold water in a strainer until water runs clear. Transfer to a large pot, cover with cold water, add 1 tablespoon fine sea salt and 1 bay leaf, then bring to a boil. Reduce to a simmer and cook until potatoes are fork-tender, about 15–20 minutes. Drain potatoes in a strainer, ensuring all excess moisture is removed. Let dry for about 3 minutes and discard bay leaf.
As potatoes dry in strainer, melt heavy cream, sour cream, and butter in the pot on low heat, stirring to combine. While potatoes are still hot, put them through a ricer and add on top of butter/sour cream mixture. Stir with a wooden spoon until smooth. 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.
Use a fork to add marks into the surface of potatoes. Drizzle with remaining 1/2 tablespoon olive oil.
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.
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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.Storage: Store cooked pie covered in the fridge for 3–5 days. You can freeze it tightly covered for up to 2 months; bake frozen pie at 350°F for 1 hour before serving.