Finely chop the onion. Add chopped onions, oil, salt, and pepper to the bowl of a large slow cooker, cover, and turn of high while you prepare the rest of the veggies.
1 large onion, 3 tablespoons olive oil, 1 ½ teaspoons kosher salt, ½ teaspoon freshly ground black pepper
Chop the potatoes into ½-inch dice. Make sure they are all fairly uniform in size so they cook at about the same rate.
3 medium Yukon gold potatoes
Peel and chop carrots into ¼-inch thick slices.
6 medium carrots
Add chopped potatoes, carrots, and broth to the slow cooker. Cover and turn to low. Cook on low for 6 to 8 hours.
3 cups vegetable broth
After filling has cooked for 6-8 hours, and veggies are tender.
In a microwave safe bowl, melt butter in the microwave, about 30 seconds. Stir the ½ cup of flour into the butter, and mix the 1 ½ cups milk, dried thyme, garlic powder, and wine into flour. Stir with a whisk until most of the lumps have dissolved. Don’t worry if you have some left, they will disappear into the pot pie.
1 tablespoon unsalted butter, ½ cup unbleached all-purpose flour, 1 ½ cups whole milk, 1 teaspoon dried thyme, 2 teaspoons garlic powder, ¼ cup dry white wine
Stir the milk-flour mixture into the veggies in the slow cooker, fully mixing it in.
Make biscuit dumpling crust
In a medium bowl, mix together the flour, sugar, baking powder and salt. Add the buttermilk to the dry ingredients in the bowl, and mix until a loose sticky batter forms, and all ingredients are combined.