Preheat oven to 350F. Coat a 10-inch cast-iron skillet (or similar-size 2-quart baking dish) with cooking spray.

Heat 2 tablespoons oil in a large pot over medium heat. Add onion, carrot and celery and cook, stirring occasionally, until the vegetables are soft, 4 to 6 minutes.

Add garlic, thyme and rosemary; cook, stirring, until fragrant, about 30 seconds.

Add spinach; cook, stirring often, until tender and wilted. Sprinkle with 1/4 cup flour and cook, stirring, for 30 seconds. Stir in broth, increase heat to high and bring to a boil. Reduce heat to a simmer and cook, stirring, until thickened, about 2 minutes.

Stir in beans and 1/2 teaspoon each salt and pepper. Transfer the mixture to the prepared pan.

To prepare biscuits: Whisk flour, baking powder, baking soda, salt and pepper in a large bowl. Using a pastry blender or your fingertips, cut or rub butter into the dry ingredients. Stir in chives. Add buttermilk and stir until just combined. Form the dough into 6 biscuits and place on top of the vegetable mixture. Lightly brush with the remaining 2 teaspoons oil.

Place the potpie on a baking sheet. Bake until the biscuits are lightly browned and the filling is bubbling, about 30 minutes. Let cool 5 minutes before serving.