Roasting the vegetables first makes this potpie stand out from others of its kind.
Heat oven to 425 degrees. Place squash, celeriac, carrots, cauliflowers, brussels sprouts, beet, and turnip or parsnips in a large roasting pan. Toss with 2 tablespoons olive oil, 1/2 teaspoon salt, and 1/8 teaspoon pepper. Roast for 35 to 40 minutes, stirring twice during the cooking. Remove, and reduce oven to 375 degrees.
Heat remaining 2 tablespoons olive oil in a medium skillet over medium heat. Add garlic and shallots; saute; until soft, about 5 minutes. Raise heat to high, and add wine. Let white wine reduce by half, 1 to 2 minutes. Add vegetable broth, and simmer over medium-high heat for 5 minutes. Set aside.
Combine 1 cup flour, baking powder, remaining 1/4 teaspoon salt, and 2 teaspoons tarragon in a food processor. Pulse in butter until mixture resembles coarse meal. Add milk and Parmesan, process until combined, and set aside.
Transfer vegetables to a large bowl. Add zucchini, remaining 2 tablespoons flour, and 2 tablespoons tarragon; toss to combine. Stir in broth, and adjust seasoning with salt and pepper to taste.
Transfer to a shallow glass pie dish, and bake for 15 minutes. Remove vegetables from the oven, and drop heaping tablespoons of the biscuit dough over vegetables, leaving some vegetables exposed. Place the pie dish on a baking sheet, and bake until biscuits are golden, about 25 minutes.
Use any combination of white, yellow, or green cauliflower in this recipe.