Onions may taste good but aren't packed with nutrients like some others are...and they can stimulate carb cravings.
I like to put shredded (or canned) carrots in my tuna (higher in carbs but needed some days)...Chopped brocoli, diced peppers, dice an avacado, pineapple chunks and shredded cheese. Mix it with salsa, corn and brocoli.
I'm not sure of your carb intake though? So I guess it depends on your diet.
Cruceferous (brocoli, cauliflower)
Green beans
Zuccini and yellow squash
green peas
carrots
spinach lettuce
leaks
peppers
Most can be bought frozen. Preferably fresh though and at least get the no salt added if you buy canned. (Carrots are actually utilized better if they're canned, otherwise their too tough to be broken down in your body)