In a 12-inch oven-safe skillet (very important that it’s oven-safe!), melt butter over medium heat. Add diced onion and saute until transparent, about 5 minutes. Add flour and continue to saute another minute. Pour in milk and vegetable broth and whisk until smooth. Add salt, thyme, sage, and some black pepper.
Bring the mixture to a boil and let simmer for a couple of minutes until it thickens to to the point where a utensil dragged through it leaves a trail. Add frozen vegetables and stir to combine. Continue to cook until veggies are heated through. Preheat the oven to 425 degrees while you make the biscuits.
For the biscuits:
In a large bowl, combine flour, baking powder, and salt. Cut in cold butter in small pieces and mix with your hands, the back of a fork, or a pastry cutter until the mixture looks like damp sand. Add cheddar and chives, then milk. Stir just until a dough comes together.
Take the veggie filling off the heat and dollop the biscuit mixture evenly across the top of it.
Bake 18-20 minutes or until biscuits are cooked through. Serve immediately, being very careful not to burn yourself as you serve from the skillet! (Lesson learned from experience.)