Heat oven to 350 degrees F. Grease a 2-quart oven safe baking dish and set aside.
Heat a large saucepan on the stove over medium heat. Melt butter, then add onions, carrots and celery. Cook until vegetables are softened, 6-8 minutes. Add garlic and cook for 1 minute, then whisk in peas, parsley, thyme and flour and cook until golden, about 2-3 minutes.
Whisk in chicken broth, then the half and half or milk and bring to a boil. Stirring frequently, simmer until thickened, about 5-6 minutes.
Remove from heat and stir in salt, pepper, diced chicken and vegetables.
Pour the chicken mixture into a greased 2-quart casserole dish. Arrange the uncooked biscuits on top.
In a small bowl whisk together the egg yolk and water, then brush over the biscuits.
Bake for 25-30 minutes, or until the biscuits are golden brown and cooked through. Let sit for 5-10 minutes, then garnish with chopped parsley and serve.
Notes
Make Ahead Instructions
The filling can be prepared up to 2 days in advance. Let cool completely and store, tightly covered, in the refrigerator.
Biscuits - For best results, you'll want to prepare the biscuits just before baking, but the biscuits can be prepped up to 2 days in advance and stored (unbaked) in the fridge.
To prepare, warm the filling in an oven safe skillet. If it's too thick, you can add a splash of milk. Top with the biscuits and bake as directed. You can also transfer to a baking dish if desired.
How to Store LeftoversStorage - Store in an airtight container in the fridge for up to 5 days. For best results, I like to separate the biscuits from the filling when storing.Reheating - For best results, reheat the filling in a skillet and the biscuits in a 350 degrees F oven for about 5 minutes. Or you can reheat the whole thing in the oven