Add the celery, onion, carrots, thyme, sage, ¼ teaspoon of salt, and black pepper to a food processor and pulse until finely chopped.
Heat olive oil in a large skillet over medium heat and add the finely chopped vegetables from the food processor. Cook for about 10-12 minutes until softened, then remove from heat and let cool.
As the vegetables cool, preheat your oven to 425°F (220°C). In a large bowl, combine bread flour, baking powder, baking soda, sugar, and remaining ½ teaspoon of salt.
In a separate small bowl, whisk together the buttermilk and 4 tablespoons of melted butter until well blended.
Add the wet mixture to the dry ingredients and stir until a dough forms. Fold in the cooled vegetable mixture gently before placing heaping spoonfuls of dough onto a baking stone or lined baking sheet.
Bake for the first round for about 12 minutes; meanwhile mix remaining melted butter with chopped rosemary. Once baked, brush this mixture onto biscuits and return them to bake for an additional 6-8 minutes until golden brown.