Drain the canned peas and corn in a colander. Rinse gently with cool water, then let them drain for several minutes so the dressing will not become watery.

Finely dice the red and green bell peppers, celery, and red onion into similar-size pieces. Drain the pimientos and chop the parsley.

In a small bowl, whisk apple cider vinegar and sugar until most of the sugar dissolves. Whisk in Dijon mustard, kosher salt, and black pepper. Slowly whisk in the olive oil to make a loose vinaigrette.

Add peas, corn, bell peppers, celery, red onion, and pimientos to a large mixing bowl. Fold together carefully to keep the peas intact.

Pour the vinaigrette over the vegetables and fold gently until everything is coated. Stir in the chopped parsley, reserving a little for garnish.

Cover and refrigerate for at least 60 minutes so the flavors mingle. For a stronger marinated flavor, refrigerate for 2 to 12 hours, stirring once if possible.

Stir gently just before serving. Taste and adjust salt or vinegar if needed. Transfer to a serving bowl using a slotted spoon if you prefer less liquid, then garnish with parsley and serve chilled.