i don't think those are lame questions at all!
i think if you are really worried about allergies, you should wait until one year. that being said, from what i know:
tomatoes are ok from 9mo (nightshade family w/ potatoes, etc.)...
a splash of lemon juice in cooking is ok, it's not the same as feeding lo an orange...
peanut butter is a definite no-no, wait until lo is at least 4-5yo...
soy sauce and cilantro, i have no idea...
fish is a tough call, tuna and milder freshwater fish is probably ok as long as it is not too often (mercury is also not in all fish), but i would avoid shellfish for a few years.
i agree with tyler's mommy that children usually don't inherit specific allergies, but rather the tendency to be allergic. so i would think that onions are probably ok, and focus on staying away from the more common allergenic foods (milk, nuts, etc.)