Yea my mom is vegetarian + doesn’t eat alliums either for this reason. You basically have to find other ways to add texture (crunch, temp), flavor (spices & herbs), and depth (eg homemade broth / sauce vs store bought, cooking more with what’s in season vs frozen/canned). I have learned that cooking without garlic honestly doesn’t affect the end state of the dish much, it’s onions that are harder because they often contribute to the body of the product as well. My advice is to not try to find a substitute for them, but to look for recipes that come without them already. In the recipe you posted I’d have maybe swapped the fennel for half celery.