How to Make It
Place peppercorns, thyme sprigs, garlic, and bay leaf in the center of a 6-inch square piece of cheesecloth. Bring corners together, and tie bundle with kitchen twine to form a sachet. Place sachet, 8 cups water, peas, and 3/4 teaspoon salt in a large Dutch oven; bring to a boil over high heat. Reduce heat to medium-low; cover and simmer 30 minutes or until peas are tender. Stir in 3/4 teaspoon salt. Remove pan from heat; cool to room temperature in cooking liquid. Remove sachet; discard. Drain (do not rinse peas).
Combine peas, celery, onion, and parsley in a large bowl. Combine remaining 1/2 teaspoon salt, oil, vinegar, and Dijon mustard in a small bowl, stirring with a whisk. Add vinegar mixture to pea mixture; toss to coat. Sprinkle with ground black pepper.