Have you looked at Deborah Madison's Vegetarian Cooking for Everyone? It's a wonderful cookbook and has pretty much everything you need to know about cooking vegetables and vegetarian meals. I recommend the book because if you flip through it, you might get some ideas for recipes to make in large batches.
Just off the top of my head, if you are cooking large batches - canelloni, lasagna, lentil soup, bean burgers - these are all things nice to have in the freezer. These ttems you can defrost quickly and serve with a green salad and crusty bread. My top favourite lentil soup recipe is one I took from Sophia Loren's cookbook! It's easy, fast and delicious but it contains anchovies (maybe you can eliminate if you are strict vegetarian). The anchovies give the soup zest but not fishy at all. Let me know if you'd like the recipe! It takes about 30 minutes, I think, to make the soup and you could make a few batches up and freeze.
If you're interested in saving time, I find I can make many many quick pasta recipes using things like parsley, lemon, red pepper flakes and olive oil within 15 minutes! There are a lot of recipes in Madison's book like this .... my husband marvels at how quickly I can whip together a tasty dish that is healthy and economical!
Last night, I made a chickpea tagine on couscous which I swear took only 15 minutes from start to finish. Again, another recipe I modified from Madison's cookbook. She has some many recipes for things, I'd not even thought about - various gratins, for instance, which would freeze well and easily be made in batches.
I know some of these tips are not for once a month cooking but I wondered if they might help anyway - assuming you're trying to save time. Hope this helps.