Vegetables and meat need to be cooked before baking, so chop any uncooked items and sauté them with a little butter or olive oil. The amounts you choose can vary widely, and remember that most will shrink after cooking, so it’s hard to overdo it. A small or medium onion and a garlic clove or two are always a good start, and then think about what you have left in your fridge that might work well together. A half pound of sausage and a bunch of spinach (cook the sausage first, and then add the greens to wilt them), or a package of mushrooms with bacon and Swiss cheese, or leftover squash with a small bunch of Swiss chard, or cherry tomatoes with corn cut off of a couple of cobs and blobs of ricotta. (Note that leftover cooked meats and vegetables don’t need to be cooked again.)