Place a 4-quart saucepan over medium-high heat and add the butter. Let the butter melt. Stir to toast and lightly brown the butter, about 7 minutes. Add the diced onion, garlic and celery. Using a spoon, stir to incorporate the vegetables into the butter. Add salt, pepper and fresh thyme. Cover and reduce heat to medium. Cook until vegetables are translucent, about 10 minutes. Once vegetables are cooked and translucent, add the frozen peas. Stir to incorporate. Turn off the heat, cover the pot and set aside.