Heat the butter over low heat and stir in the onions. Cook until the onions are softened.
Whisk in the flour. Add the broth, bay leaves, and thyme, and bring to a boil.
Stir in the celery, carrots and potatoes. Cover and bring to a boil again. Reduce heat to a simmer and cook until the potatoes are almost done. Add the peas.
Meanwhile, combind the flour, baking powder and salt in a large bowl. Cut in the butter with a fork and then stir in the milk. Mix until just combined.
Drop heaping tablespoons of dough into the veggie mixture. Cover the pot and cook for about 10 minutes, or until the dumplings are cooked through.