In a large stock pot, melt 1 1/2 Tbsp. of the butter over medium-high heat. Sautee the onions, carrots, and celery for about 3 minutes. Add the broth, potatoes, and seasonings, and bring to a boil. Reduce the heat to medium, press the veggies down into the broth (making extra sure the potatoes are all covered with liquid), and cover. Cook 15-20 minutes until the potatoes are tender, stirring occasionally. Reduce the heat to low and stir in the peas.