Vegetable Pot Pie with Cheddar Scallion Biscuit Topping

We like pot pie. I've made one before, but I like to try different recipes. Tonight, we decided to try this recipe from Playing House. You will not believe the smell that this pot pie fills your house with as it cooks! It is amazing. So is the taste!

1 (1 pound) package Frozen Pearl Onions
2 1/2 cups Water
5 tablespoons Unsalted Butter (*I only used 3)
1 Thyme Sprig + 1/2 teaspoon Thyme Leaves (*I didn't use the sprig)
Salt & Pepper
1 Medium Onion, coarsley chopped (*I omitted)
3 Carrots, coarsely chopped
2 Celery Ribs, coarsely chopped
1 Parsnip, coarsely chopped (*I omitted)
1 Medium Potato, peeled and coarsely chopped
10 ounces White Mushrooms, trimmed & quartered (*I used sliced)
1 cup Whole Milk (*I used skim)
1 (10 ounce) package Frozen Peas, thawed

Biscuit Topping:
2 1/4 cup Flour
2 teaspoons Baking Powder
1 teaspoon Baking Soda
3/4 teaspoon Salt
1/4 teaspoon Pepper
6 tablespoons Unsalted Butter, chilled & cut into 1/2 inch cubes
1 cup Extra Sharp Cheddar, coarsely grated
3 tablespoons Parmesan Cheese, grated
2 Scallions, chopped
1 1/2 cup Buttermilk, well shaken

Combine pearl onions, water, 1 tablespoon butter, thyme spring, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a saucepan and bring to a boil. Reduce heat and simmer, covered, about 8 minutes or until the onions are tender. Drain and reserve the liquid, discarding the thyme sprig. Meanwhile, heat the remaining 1/2 stick butter (I used 1/4 stick) in a 12-inch deep heavy skillet (I used a wok-the only thing we have that is big enough!) over moderately high heat until the foam subsides. Add the chopped vegetables, mushrooms, garlic, thyme, and 1/2 teaspoon salt, stirring occasionally, until the vegetables begin to brown, about 15 minutes. Sprinkle vegetables with flour and cook, stirring, for 1 minute. Add the reserved onion cooking liquid and milk and bring to a boil, stirring and scraping up any brown bits. Reduce the heat to moderate and simmer, covered, for about 10 minutes, or until the vegetables are barely tender. Remove from heat and stir in the peas, pearl onions, and salt and pepper to taste. Pour the vegetable mix into a 9X13 baking dish.

To make the biscuits, whisk together the flour, baking powder, baking soda, salt, and pepper in a large bowl. Blend in the butter with your fingertips or a pastry blender until the mixture resembles coarse meal. Stir in the cheeses and scallions. Add the buttermilk and stir until just combined. Drop the biscuits into the filling in 12 equal mounds, leaving spaces between each. Place the baking dish on a foil lined pan and bake at 450 degrees for 25-30 minutes, or until the topping is golden and the filling is bubbling. Let it stand for 5 minutes before serving.