In a large bowl, cover peas with 4 inches of water and soak a minimum of 6 hours, or overnight. Drain and rinse thoroughly.
Place peas in a 4-quart saucepan with chicken broth or water and bring to a boil. Reduce heat and simmer peas between 45 to 55 minutes, or until peas are cooked but not mushy. Drain excess liquid and let peas cool to room temperature.
Combine buttermilk, mayonnaise, mustard, vinegar, hot sauce salt and pepper in a bowl, and whisk to incorporate.
In a salad bowl, combine beans, red onion, celery and chives. Pour dressing over bean mixture and gently stir to incorporate, being careful not to break up too many of the beans. Add spinach and toss to coat. Season with additional salt, pepper and hot sauce, to taste. Serve immediately.