Make sure that the butter is chilled; otherwise the texture of the biscuits won't come out right
You can also use a fork; however, it's more work, and you might also bend the fork.
Preheat oven to 425° F.
Mix 4 cups flour, 2 Tbsp baking powder, and 2 tsp salt in a large bowl.
Using a pastry cutter or fork, cut the butter into the flour mixture until it resembles coarse crumbs.
Slowly stir milk into the dough, a bit at a time until the dough comes together. Stop adding milk when the dough is just able to hold itself in a cohesive ball.
Too much milk will cause the dough to become sticky and impossible to work with.
If you haven't already, grate ¾ cup of sharp cheddar cheese. Add the cheese and ⅓ cup chopped fresh chives to the dough and mix to combine.
Lay out dough on floured surface until it's about the size of the casserole dish. Stretch the dough lengthwise until it is twice the size of the casserole dish, then fold in half. Repeat several times to form layers.
Apply non-stick spray to a baking sheet, or cover with silicone baking mat. Cut 6 biscuits from the dough and lay on sheet. Arrange them to form a rectangle approximately the size of the casserole dish. If necessary or desired, cut smaller pieces of dough to fill gaps.
Bake biscuits at 425° for 8 minutes. Remove from oven, but leave oven on. Also, do not eat the biscuits yet, as they are not fully baked.
Microwave the pearl onions for about three-quarters of the time suggested on the bag.
Boil a pot of water. Chop 1 pound of petite red potatoes into bite-sized pieces. Place the potatoes into the pot and boil for 15 minutes. Drain when done boiling.
Heat 2 tablespoons of olive oil over medium in the large skillet. Press 2 cloves of garlic into the pan if you're using a garlic press; otherwise finely chop the garlic and add it to the pan. Add the onions to the pan and sauté until your kitchen starts to smell good.
Add the bag of frozen peas and carrots to the pan and cook until heated.
Add ½ tsp black pepper, ¼ tsp thyme, and ½ tsp salt to onions and garlic; stir.
If you haven't already, measure out 1¾ cup vegetable broth and ⅔ cup milk, and keep it at hand near the stove. Having this ready will help prevent the flour from burning onto the bottom of the pan.
Spoon in ⅓ cup flour and stir until the vegetables are evenly coated. Then, immediately pour in the vegetable broth and the milk.
Bring to a boil, then simmer until the gravy has thickened, stirring occasionally. Make sure you scrape the bottom of the pan every now and then to prevent the gravy from burning to the bottom of the pan. You'll know it's thick enough when it drips slowly from a wooden spoon.
Pour the filling into the casserole dish. Mix the potatoes throughout.
Cover the top of the casserole dish with biscuits, and bake for another 7-8 minutes until the biscuits are fully cooked.