Add the farro to a medium pot and cover with a few inches of salted water. Bring to a boil, then reduce the heat to simmer, and cook uncovered until tender. (Cooking time will vary depending on the type- pearled farro cooks the fastest in about 15-20 minutes). Drain and let cool slightly.

While the farro cooks, microwave the peas until thawed. Toast the walnuts in a dry skillet or in the oven. Slice the watermelon radishes into thin matchsticks and roughly chop the parsley. If using preserved lemon, scoop out and discard the flesh, then finely chop the peel.

In a small jar, whisk together the miso and the lemon juice until smooth. Add the olive oil and salt, then cover and shake until emulsified.

Add the warm farro to a large bowl along with the peas, walnuts, radish, parsley, and preserved lemon (if using). Pour over the dressing and toss to combine. Taste and adjust with more salt if needed.