Can I have this dish without garlic?
A-glio — the 'gl' is a palatal lateral, like 'lli' in 'million'. Two syllables, stress on the first.
When you want to modify a dish to remove a specific ingredient you dislike or are allergic to.
'Senza' means 'without'. You can replace 'aglio' with any ingredient: 'cipolla' (onion), 'peperoncino' (chilli), 'prezzemolo' (parsley), etc. Some modifications are easy; others may change the dish fundamentally.
È possibile non mettere la cipolla?
Is it possible to leave out the onion?
Alternative phrasing — 'non mettere' (not put in) is very natural
Potete prepararlo senza peperoncino?
Can you prepare it without chilli?
Specifying heat intolerance — common request in spicy regional dishes
Posso averlo senza formaggio?
Can I have it without cheese?
Useful for dairy-free requests or lactose intolerance
Modifications to Italian dishes are generally accepted with grace in tourist-friendly restaurants. However, in very traditional trattorias, asking to remove a key ingredient from a classic recipe (like garlic from aglio e olio) may get a surprised look — the recipe is the recipe.