How to Make It
Stir together peas, onions, carrot, celery, garlic, bay leaves, parsley, and salt in a medium saucepan. Add stock to cover by a depth of 1 inch. Bring to a boil over high, skimming foam that rises to the top. Reduce heat to medium. Partially cover; simmer until tender, 15 to 25 minutes. Remove pan from heat; let stand 15 minutes.
Remove and discard onions, carrot, celery, garlic, bay leaves, and parsley. Do not drain peas. (To make ahead, transfer cooked peas and liquid to an airtight container; cover and refrigerate up to 3 days.) Stir in butter, thyme, and pepper. Serve warm.