Heat the vegetable oil in a large pot over medium-high heat. Add the beef stew meat and brown it on all sides for about 5 minutes.
Stir in the onion, carrots, and celery. Cook for 3 minutes until the onion softens.
Sprinkle the 2 tablespoons of flour over the mixture and stir for 1 minute to coat everything.
Pour in the beef broth, then add the potatoes, thyme, 1 teaspoon salt, and black pepper. Bring to a simmer, cover, and cook for 45 minutes until the beef is tender.
While the stew simmers, mix the 1.5 cups flour, baking powder, and 1/2 teaspoon salt in a bowl. Stir in the buttermilk until a soft dough forms.
Drop spoonfuls of the dough onto the simmering stew. Cover the pot and cook for 15 minutes without lifting the lid so the dumplings steam properly.