After my onion seedlings came up, and while they were still in flats,
I gave them haircuts to stimulate bulb growth. Now that they're in
the gaden, shouldn't I be doing the same thing? I don't see any
reason I can't clip them to about 4 or 5 inches and let all that
energy go into bulb formation.

Any thoughts?

What energy? It is the tops that convert sunlight into energy. Plants don't do
well if you remove too many of their leaves.