1. He threw food onto my face.
2. He threw food on my face.
They're both grammatically correct.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
I think it's most common to hear "in" my face, however, as in:
He spit in my face.
Batwing1. He threw food onto my face.
2. He threw food on my face.
In these the food made contact with the face.
If you mean only that he attempted to strike your face with the food, whether it found its target or not:
He threw food at my face.
CJ