Preheat oven to 425℉.

Divide the diced butternut squash, pumpkin, carrots, Brussels sprouts, and cauliflower between two large baking sheets. Drizzle with olive oil and season with salt and black pepper. Toss to coat and roast in the oven for about 20 to 25 minutes until golden brown and crispy, tossing halfway through. Remove and cool on baking sheets. Lower oven temperature to 375℉.

Heat a large heavy-duty pot over medium-high heat. Melt 2 tablespoons of butter, then add the chopped onions and garlic. Sauté for about 5 minutes until onions soften. Season with salt, black pepper, and crushed red pepper flakes. Cook until caramelized, about another 5 minutes.

Add sliced mushrooms and cook until browned, about 5 minutes. Stir in chopped thyme, rosemary, and bay leaves, then cook for an additional 5 minutes. Move veggies to one side, add remaining 1 tablespoon butter to the pot, melt it, and then add flour. Stir and cook for about 30 seconds.

Slowly add vegetable stock while stirring to combine. Lower heat and allow to thicken for about 10 to 15 minutes, checking for seasoning. Stir in roasted vegetables, Greek yogurt, and frozen peas, and cook for another minute. Remove from heat and allow to cool slightly.

In a large bowl, whisk together flour, baking powder, salt, black pepper, and rosemary. Add cold butter and rub with fingers until mixture resembles coarse crumbs.

In a small bowl, whisk together egg yolks and buttermilk. Add to flour mixture and stir until dough comes together. Shape into a disk, cover with plastic wrap, and chill for at least 1 hour.

On a lightly floured surface, cut dough into four pieces. Roll out each piece into a circle, trimming edges for a clean circle. Divide filling among ramekins or baking dishes, mounding it up. Top each pot pie with the dough, brushing edges with egg wash to seal.

Place pot pies on a baking sheet. Make small slits on top crusts for steam to escape. Brush with egg wash and sprinkle with freshly cracked black pepper. Bake for 25 to 30 minutes until golden brown. Remove from oven and allow to cool slightly before serving.