Is my peach tart going to be awful because my peaches are slightly under ripe? :(

Hi Lacy, sorry I didn't answer sooner -- was traveling. The above answers are great; while ripe peaches are best for this recipe, it's a pretty forgiving tart and the topping would probably mask most underripe ills. Hope all turned out well!

If you can't wait, slice the peaches and macerate them in (fresh!) lemon juice for a couple of hours. Drain and dry them before using them in the tart.

Can you wait a few days to make it? If so, toss them in a paper bag and let them ripen on the counter. If not, you could try roasting them slightly before using.