I usually omit the fennel just because I don't have it on hand at the time and it still turns out delicious. If you are sensitive to the acid of vinegar, just don't use as much of the dressing, but I think it's perfect. I normally don't have Dijon so plain yellow mustard works perfectly fine as well. I like the spice so I always use some sort of hot sausage. Sometimes when I don't have any apples I also substitute dried cranberries instead.