There are a few different ways you can do it.
The first options is to cook vegetarian side dishes, then just serve a different protein for the vegetarians and meat-eaters. You’ll all be eating 90% the same, so there won’t be too many additional pans to wash. I know a lot of vegetarians aren’t fond of meat substitutes, but they’re really useful for this kind of meal. Veggie sausages, soya ‘chicken’ nuggets, grilled Quorn ‘steaks’ – they can all be used as one-for-one replacements for their meaty equivalents.
I’d do this for roast dinners, burgers, hot dogs, bangers and mash.
Another option is to cook a vegetarian meal, and then scatter some kind of meat or fish on top for those who want it. I sometimes find it helpful to do a roast/pulled chicken or roast/pulled pork once a week so that we have some cooked meat on hand for adding in at the end. You can add pulled chicken right at the end to: enchiladas, fajitas and stuffed peppers. Roast chicken can be added at the end to a chickpea and couscous salad or tagine. I add the cooked pork at the end to dishes like curries and stir fries. It's also useful to have bacon lardons for frying and adding right at the end to egg fried rice, carbonara, risotto and lentil casserole.
Make a bunch of beef meatballs and keep them in the freezer, so that a veggie tomato sauce for pasta can get a beefy boost when needed. Also, lamb meatballs are great to throw into a chickpea and vegetable tagine over some couscous, or can be part of a tapas or mezze dinner.
Another option is to use two separate pans, which might sound like the equivalent of making two separate meals, but it’s not! If you’re making a stew or casserole, you can make the exact same thing in two different pans, with barely any extra effort. Chop all your veg, and rather than dumping them into one pan, separate them into two. All the other ingredients can be added to both pans, with meat added to one, and some kind of veggie protein added to the other. Sure, you’ll have two pans to wash instead of one, but there’s otherwise the same amount of prep work and cooking time. It's easy to make two small toad in the holes from one batter mix and just use different sausages. This oven-baked frittata is a good veggie dish if you fry the bacon separately and use two separate oven dishes, leaving the bacon out of one of them www.bbcgoodfood.com/recipes/3077673/bacon-and-ricotta-ovenbaked-frittata
It’s worth making a big batch of these kinds of meals. By the time you’ve started prepping, it’s not really any extra effort to make a larger amount. Stews and soups freeze really well, so you can freeze any leftovers in portions, to make for a quick and easy dinner another night.