If using frozen peas, thaw them under cold water and drain.
In a large mixing bowl, combine the peas and diced cucumber.
Add the finely chopped red onion to the bowl.
In a small bowl, mix the mayonnaise and lemon juice until smooth.
Pour the dressing over the pea mixture.
Season with salt and pepper to taste.
If using, add the chopped fresh dill and feta cheese.
Toss everything gently until well combined.
Cover and refrigerate for at least 30 minutes to let the flavors meld.
Serve chilled and enjoy your vibrant pea salad!