In a large stockpot, bring 4 cups of water, salt, pepper, and the peas to a boil. Once the peas reach a boil, lower to a simmer and cook for 30 minutes or until the peas are done. Let cool.
In a large bowl, gently mix black-eyed peas, red onions, tomatoes, cilantro, garlic, mustard, lemon juice, and olive oil.
Mix spinach gently into black-eyed pea mixture. Serve at room temperature.