For Sunday lunch my fave 'easy but impressive' is Sicilian lamb.
Take a leg of lamb, brown it and put to one side. Soften some chopped onions, add two or three chopped garlic cloves and then two tins of chopped tomatoes, two dessert spoons of balsamic vinegar and two dessert spoons of sugar, mix and heat through. Put lamb in a roasting tin, add tomato mixture, cover with foil and then roast as per normal lamb, basting with the tomato sauce.. Take off the foil for the last five minutes or so.
Once you have uncovered the lamb, cook some pasta (can be spaghetti or ... any type really). Remove lamb from the oven, and set aside to rest. Once the pasta is cooked, drain it, add to the tomato sauce from the roast and mix thoroughly.
Slice the lamb, then serve with the pasta, bit of parmesan and a green salad.
Dead easy and different. I tend to put all of it into the middle of the table and just let people help themselves, that way, rather than put all of it on a plate at once, people can have some pasta, then meat and salad or any variation they fancy. It's also worth saying that it's a favourite of DD and DS!