Add the frozen peas to a measuring cup. Set aside to defrost at room temperature.
1 cup frozen peas
Heat the oil in a large, heavy saucepan or stockpot over medium heat.
2 tablespoons olive oil
Once the oil is hot, add the celery, carrots, and onion. Cook, without browning, until the vegetables begin to soften, about 5 minutes.
3 ribs celery, 3 medium carrots, 1 medium onion
Sprinkle the flour over the vegetables and cook, stirring constantly, until the flour is incorporated into the mixture but not browned.
¼ cup all-purpose flour
Add the butter and cook, stirring gently, until the butter melts. Slowly add the chicken stock, one cup at a time, stirring constantly, until the sauce thickens.
4 tablespoons unsalted butter, 4 cups low-sodium chicken stock
Add the shredded chicken and peas to the pot, then cook for a minute or two.
2 chicken breasts
Remove from heat. Stir in the Parmesan cheese, thyme, and salt and pepper.
2 tablespoons grated Parmesan cheese, 1 tablespoon fresh thyme leaves, ½ teaspoon Kosher salt, ¼ teaspoon freshly ground black pepper