I have 3 family members that are vegetarian, so I have been coming up with ways to adapt my recipes for them to have when they eat over. The easiest way I have found is to substitute ground beef with a mixture of two or more of the following:
Pressed, crumbled firm tofu
Cooked Barley
Cooked Brown Rice
Cooked Quinoa
finely chopped mushrooms
grated carrots and zucchini
I mix up whatever of the above I have on hand and use it exactly as I would ground beef--such as in "meatloaf", "meatballs", as taco filler, in casseroles and in lasagna, burgers, etc. So far I have had no complaints and more than a few compliments, even from the meat-eaters.