Arrange the salad mix among 4 bowls, piling it high. Top each with the cucumber, red onion, and cheese. Scatter the tomatoes, olives, and pepperoncini over each salad. Drizzle each salad with 2 tablespoons of Greek dressing.
In a blender, combine the vinegars, mustard, salt, pepper, red pepper, and garlic and puree. With the motor running, very slowly pour in a steady stream of olive oil to make a smooth dressing. Repeat with the canola oil.
Transfer the dressing to a glass storage container and mix in the oregano. Taste the dressing and season with salt, if necessary. The dressing will keep for 2 weeks, covered in the refrigerator.
Recipe courtesy George Stella, George Stella's Livin' Low Carb: Family Recipes Stella-Style, Simon and Schuster, 2005