I thought I had useful suggestions, but the PP's pretty much have it covered. All good options; between them I like the rotisserie chicken idea. I'd be afraid of wasting the beef too. At the same time, I'm sympathetic to DH's desire to rescue his earlier efforts and not pitch them. Using roti chicken is a lower-effort option.

Only other idea: If you're going to fry anything to add in (be it beef, some new onions, whatever), do the frying in a little bacon grease if you have it.

One of the benefits of stewing long and slow is to get a richer blend of flavors throughout. At this point, the veggies have been cooked without the flavors melding in. If you want to try this, I would try to amp up the flavor of the beef as you cook it. For example, Cook the beef and add rich flavors such as soy sauce or worcestershire sauce. If you want to spend the money, you can buy beef demi-glace which will add a lot of beef flavor, but it is expensive. Our local supermarket has this product called Better than Bouillon which is a paste-like soup base that is much stronger in flavor than bouillon and great for something like this http://deals-on-call.amazonwebstore.com/Superior-Touch-Better-Than-Bouillon-Beef/M/B00016LA7U.htm. Make sure that you cook the beef long enough to get past the tough stage (stews are best when the meat softens again and is fall off the bone or fall apart soft) and then make sure there is ample flavor. I would then just mix one bowl of veggies and beef to see how well the flavors meld. If they don't, serve the beef over grain of some sort (it would probably be great over polenta).

Go easy on yourself and shred some chicken off a rotisserie chicken from the store instead. Add a spoonful of vinegar and a spoonful of sugar or agave, salt, pepper, and a teeny bit of cayenne or hot sauce. The sour/sweet/salty/spicy mix will make the stew taste interesting again. If you want throw in a little bit of chopped herbs or a cube of frozen basil.

Dump the vegetables - the nutrition has been cooked out of them.

Agree with PP, but honestly you're probably better off just starting over. I'd be afraid of wasting the beef.

I would season and brown the meat, deglaze with wine (or some of the liquid from the stew), sprinkle some flour on top and then braise it in the oven in either beef broth or strained liquid from the stew. I would go with broth if the stew is really bland. When the meat is done, add the strained veg from the original stew and enough of the liquid to get the consistency you want. If that's more work than you want to do, I would at least brown and deglaze the meat, and finish cooking the meat in the liquid from the broth. Then add it to the already-cooked veg.

My husband made a veggie stew that just wasn't very flavorful. We have another dinner's worth left over, so he's planning on adding beef to it to see if he can make it better. The only thing is, we're not sure how/when to add the beef... the veggies in the stew are already cooked. Would you just fry up the beef and toss it in? Stew the beef in broth and mix it in?