Here’s my logic. Onions are a low-acid food. To make them safe for water-bath canning, you have to add sufficient vinegar to drive the pH below 4.6. The question is whether there is enough vinegar in this recipe. If you have pH strips handy, then you could just find out, and proceed accordingly. If you don’t, however, you have to do some sleuthing.

Two things made me nervous. First, while Joy of Cooking includes a similar recipe for what they call “red onion marmalade,” it’s not included in their list of condiments safe for canning. But it was what I found in the Ball Blue Book that really convinced me. The Blue Book includes two different recipes for preserved onions: one for water-bath canning, and one for pressure canning. Their red onion marmalade, safe for water-bath canning, has a ratio of 1 1/2 c. onions to 1/4 c. vinegar, plus some orange peel and a whole lot (4 cups!) of sugar. The red onions with honey recipe, on the other hand, has 2 1/2 quarts onions, 1/2 c. wine, and 1 c. honey, and must be pressure cooked. Serious Eats’ confiturra has about 10 cups of onions, 3/4 c. vinegar, and 1/2 c. wine, but by the Blue Book logic, you would need 1 2/3 c. vinegar for 10 cups of onions. Given that the Blue Book is known to be conservative, it strikes me as within the realm of the possible for 3/4 c. of vinegar to be enough, but since I’m giving these as gifts, I wanted to play it safe. Fortunately, since this is a heavily cooked product anyway, there’s no harm in pressure canning the jars.

All of which is a long way of saying: if you want to preserve a low- to moderate-acid onion confit, pressure can the jars at 10 pounds for 15 minutes. If you don’t have a pressure canner, they’ll keep in the refrigerator for at least a month. As a reminder, I’ve got step-by-step pressure canning instructions here.