Is there anything that particularly benefits brussels sprouts? I've never grown them before, but I have a 2'x2' square set aside for 1 brussels sprout plant this year. I want to plant something short around it; thinking maybe beets or lettuce, but I'm wondering if there is a good companion plant.

I used to plant Pyrethrum daisies (Pyrethrum Daisy (Tanacetum cinerariifolium)) amongst my Brassicas. If you have problems with Cabbage Whites, the flowers will act as a deterrent to them. Marigolds are also a good pest-deterrent generally, but I'm not sure if they help with Cabbage Whites specifically.

Lettuce would probably be OK as an edible companion - the shade from the taller plant won't bother it at all, and you can harvest it without affecting the sprouts. I wouldn't plant a root crop like beets there for two reasons: sprouts like a firmly packed soil while root crops need it lighter, and you would disturb the roots of the sprouts if you try to harvest any beets before it's ready.

Thank you! I will look to see if I can find those daisies anywhere.