Al's highly unorthodox method of approval surprised a bark of a laugh out of Snape. The fact that Al had remembered that detail of the thoroughly awkward-bordering on disastrous-dinner, and felt able to joke about it, was eloquent testimony to Al's acceptance of him. And that fact made the memories of the whole dinner debacle much less horrid.
Besides, he genuinely liked mushy peas. Tyke that he was, he'd grown up with a taste for them with his fish'n'chips, and pies, and even on their own. He grinned and nodded to Al, and summoned parchment and quill, knowing that Lily could recognise his cramped, angular script, just like he could recognise her handwriting at a glance.
Dear Lily and James,
Thank you very much for the invitation; Al and I would love to come.
As for veg, you know me, did you even need to ask? Mushy peas are even green, how healthy can you get?
We'll be there at 6.
Severus
Instead of signing for Al, he handed the parchment and quill to him. There was plenty of space left on the parchment, if Al wanted to write his own note instead of just signing his name.