I think Heather's issue is that anyone with any skill she doesn't possess is threatening her "superiority." I would just pause, mid-tidbit, raise one eyebrow and chuckle. Then eat the rest of your meal as though she'd just told a joke. Her comments about how you eat your food are beyond ridiculous and don't deserve the attention. If she persists, look incredulous and say "I thought you were joking. I am going to eat my meal as I please." (Don't justify, argue, defend or excuse yourself whether you use a fork, chopsticks, etc.)