I just remembered that I make a version of Gennaro Contaldo's Italian lamb stew that uses both onion and garlic, though the garlic is not sautéed with the onion in the soffritto but bashed and added later. Dunno whether that's significant in preventing the two from fighting each other. It's a fantastic stew, actually, though the lamb shoulder needs longer cooking than he seems to suggest. I don't add the peas until the final heating before serving, though, as with any stew or soup, it tastes much better made the day before. There's a YouTube of Gennaro in Malta making this stew.

Gino D'Acampo is militantly opposed to using onion and garlic together. He's often been accused of blandness and over-simple recipes (as well as burglary), but his dishes never fail for me. Some other cooks are less evangelical about it, Delia and my hero Elizabeth David among them. It could be that the combination is anathema in some parts of Italy, not so much in others. I never use onions in pasta sauces as they tend to form a disagreeable sludge with the tomatoes. I prefer less sauce and more intensity, with good bronze die pasta that holds the sauce nicely. None of that slippery penne for me, though I can knock up a good arrabbiata with rigatoni, either with salmon or chicken and plenty of chilli. On the other hand I wouldn't dream of putting garlic anywhere near a risotto.