Dates help balance this salad with their sweetness.
Preheat oven to 350 degrees. Spread walnuts in a single layer on a rimmed baking sheet, and toast in oven until golden and fragrant, 10 to 12 minutes. Let cool.
In a food processor, pulse 5 dates, mustard, and vinegar until pureed. With the machine running, slowly pour oil through feed tube. Season with salt and pepper.
Quarter remaining dates lengthwise. Into a large bowl, tear lettuce and endive into bite-size pieces; add spinach, dates, and walnuts. Drizzle with vinaigrette, and toss to combine. Serve immediately.