Preheat the oven to 375°F (190°C).
In a large skillet over medium heat, melt the butter.
Add the chopped onion and cook for about 4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the flour and cook for 2 minutes, stirring constantly to form a roux.
Gradually whisk in the chicken broth and milk, continuing to whisk until the mixture thickens and comes to a simmer, about 5 minutes.
Remove the skillet from heat and stir in salt, black pepper, dried thyme, dried sage, and paprika.
Add the cooked chicken and thawed mixed vegetables to the sauce, stirring until evenly combined.
Transfer the chicken and vegetable mixture to a 9×13-inch casserole dish, spreading it out evenly.
Separate the biscuit dough into individual biscuits and arrange them on top of the chicken mixture, covering the surface.
Brush the tops of the biscuits with olive oil to promote browning.
Cover the casserole tightly with aluminum foil if freezing, or leave uncovered if baking immediately.
To freeze: Place the covered casserole in the freezer for up to 3 months.
To bake from frozen: Preheat the oven to 375°F (190°C). Remove the foil and bake the casserole for 60-70 minutes until the biscuit topping is golden brown and the filling is bubbly.
To bake fresh: Bake the uncovered casserole for 35-40 minutes until the biscuit topping is golden and the filling is hot and bubbly.
Let the casserole rest for 5 minutes before serving.