Drain the sauerkraut thoroughly in a colander. Finely chop the red onion.

In a large mixing bowl, combine the drained sauerkraut and chopped red onion. Add bell peppers and carrots if desired.

Sprinkle in salt and black pepper, then drizzle with olive oil and apple cider vinegar. Mix gently.

Cover and refrigerate for at least 2 hours, ideally overnight.

Before serving, give the salad a gentle toss and adjust seasoning if needed.