What would be the best replacement for fennel? My husband is allergic!

Just reading the list of ingredients, I'd say you could sub here and there for several things, whether it be due to an allergy in your case, or something you don't happen to have on hand. I always have a can or two of artichoke hearts (in water, because those packed in oil are so......oily) in the pantry. Whether you make your own, or buy it in jars, some pesto would be lovely swirled in. Some finely shaved cabbage - green or red) would lend a texture similar to the missing fennel. In the end, a recipe is a guide - hopefully a good one - not a legal contract. There's lots of fun to be had with this one.

Shakshuka is one of those things that is pretty versatile when it comes to what gets tossed in the sauce. In this case, the idea is to accent the green color. The fennel is being added for flavor and texture, but it's not critical to the recipe. You could use frozen sliced artichoke hearts, add in more spinach or other leafy green, and it will still taste good. You won't have the slight anise flavor of the original, but that won't mean it doesn't taste as good. Just slightly different taste. I'm not allergic, I just don't care for the taste of fennel. So I use artichoke hearts, and a time or two I've done it with matchstick sliced celeriac, and even the suggested eggplant. I mean, it's shakshuka- with cheese no less, so no matter what you toss in it's bound to be tasty.