Combine salad mix and chopped cucumber, sliced red onion, feta cheese, cored and quartered tomatoes, chopped olives and sliced pepperoncinis in salad bowl. Toss to mix and divide onto four plates.
In a blender, combine vinegars, mustard, salt, pepper, red pepper, and garlic. Puree. With blender running, slowly stream in olive oil and repeat with canola oil. Add salt and pepper to taste.
Pour dressing over salads and serve, or serve dressing along side salads.