Preheat the oven to 425°. Butter a large casserole dish. Pull the meat off the chicken, shredding it with your fingers or a fork into the baking dish.
In a large skillet, melt 3 tablespoons of the butter over high heat. Add the mushrooms, onion and celery and cook, stirring occasionally, until the mushrooms are browned, about 6 minutes. Lower the heat to medium, sprinkle the flour into the skillet and cook, stirring, for 1 minute. Stir in the chicken broth, bring to a simmer and cook, stirring, until thickened, about 2 minutes. Stir in 1/3 cup of the milk and simmer for 1 minute more. Remove the skillet from the heat, add the lemon juice, and add salt and pepper to taste. Pour the gravy over the chicken in the baking dish.
Melt the remaining 4 tablespoons of butter with the garlic.
Stir together the baking mix, the remaining 2/3 cup of milk and the cheese. Drop tablespoonfuls of the biscuit mixture on top of the casserole and brush with half of the garlic butter. Bake until the biscuits are golden brown, about 25 minutes, brushing them once or twice with the remaining garlic butter.