Prep the vegetables by peeling, scrubbing and cleaning. Dice everything into small pieces appropriately bite-sized for pot pie. Onions and most of the vegetables can be combined to go in at the same time; the potatoes should stay separate. Also grate the parmesan cheese; finely mince the rosemary; and, strip the kernels from the corn (or set aside the frozen kernels if fresh is out-of-season). Mix the 2 tsp bouillon mix and some hot water and set aside. Mix the flour and dry ingredients. Verify you have all ingredients on hand, ready to use. (A reminder for this one because it is fastest if fully prepped.)
Preheat the oven to 450ºF. Gently melt 2 tbsp butter in a Dutch oven over medium heat on the stovetop, then transfer to a medium heat-proof bowl.
In the same Dutch oven, heat 1 tbsp oil over medium high. Add the onions and vegetables. Season with salt and pepper. Cook 5-7 minutes undisturbed, until softened and developing color.
Add the 2 tsp broth concentrate and 1½ cups water. Bring to a boil. While that is coming to a boil, whisk together 2 tbsp flour and ¾ cup milk until smooth. Once the pot is boiling, stir in the corn, potatoes and flour/milk mixture. Reduce heat to low and simmer, stirring occasionally, until cream sauce thickens, 4-5 minutes (and while you prepare the biscuits.
While that cooks, add ⅔ cup milk and 1 tsp vinegar to the bowl of melted butter and stir to combine (butter will clump). Add the premixed dry ingredients and stir to combine until there are no traces of dry flour.
Spoon 6-8 large drops of biscuit dough over the vegetable filling. Bake on upper rack until the biscuits are golden and cooked through and everything is tender, about 18 minutes.