Bring a large saucepan of water to a boil over medium-high heat. Add the frozen peas and cook for 2–3 minutes, just until heated through and tender-crisp. Drain immediately in a colander and rinse under cold water until completely cool. Drain well.

Cook the bacon in a skillet over medium heat until browned and crisp, about 6–8 minutes. Transfer it to a paper towel-lined plate to drain, then chop into small pieces. Let it cool completely before adding it to the salad.

In a small bowl, whisk together the mayonnaise, sour cream, apple cider vinegar, sugar, kosher salt, and black pepper. The dressing should be smooth, creamy, and lightly tangy.

Place the cooled peas, chopped bacon, cheddar cubes, and diced red onion in a large mixing bowl. Add the optional parsley.

Pour the dressing over the pea mixture and fold gently with a spatula until the peas and other ingredients are evenly coated. Avoid vigorous stirring, which can crush the peas.

Cover the bowl and refrigerate for at least 1 hour. This gives the dressing time to coat the ingredients and allows the flavors to blend.

Stir gently before serving: Taste and adjust the seasoning with a small pinch of salt or pepper if needed. Serve chilled.