Measure and cut the butter into small pieces. Place the butter on a plate and put it in the fridge to chill while preparing the ingredients.
Gather your ingredients. Finely chop the carrots, celery, and fennel. Alternatively, you can add them to a food processor and pulse until finely chopped.
Add 1 tablespoon of butter to a skillet over medium heat. Add the chopped vegetables and cook for 5-7 minutes, until softened. Transfer the cooked vegetables to a small bowl and place in the freezer to cool for at least 10 minutes.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, sugar, and salt. Add the cold, diced butter to the mixture. Use a pastry cutter or two forks to break down the butter and incorporate it into the flour until the mixture resembles coarse crumbs.
Whisk in the chopped fresh herbs, spices, scallions, and cooked veggies.
Pour in the cold buttermilk and gently stir until just combined. Be careful not to overmix; the dough should be slightly sticky.
Sprinkle flour onto a parchment-lined baking sheet and place the dough on top. With floured hands (or floured rolling pin), pat it into a rectangle about 1 inch thick. Gently fold the dough on top of itself about 2-3 times, making sure to spread it lightly each time.
Press the dough out a final time until it’s a 1-inch thick rectangle. Place the dough in the freezer for 20 minutes to chill.
Preheat the oven to 425 °F
Spread butter onto a 7 x 10.5 baking pan or cast iron. Use a sharp knife or biscuit cutter to cut the dough into squares and place them in the baking pan leaving a little space between each biscuit.
In a small bowl, combine 1 tablespoon of buttermilk with 1 tsp chopped rosemary. Brush the tops of the biscuits with the buttermilk mixture.
Bake in the preheated oven for 22-25 minutes or until the biscuits are golden brown.
Allow the stuffing biscuits to cool for about 5-7 minutes before serving. Enjoy!