What can I use to substitute for Italian sausage to make it appropriate for vegetarians?
You can substitute the sausage meat itself with any suitable veg that's normally used as the star of a dish, such as eggplants or firmer squashes, however if you want that sausage taste the principle flavoring spices of Italian sausage are caraway seed, sage and sometimes paprika. If you want to add an umami element (meaty taste) substitute all salt with soy sauce, and perhaps add a little extra soy (experiment) to get the level of taste you want without making the lasagna over watery.