Vegetables are not garnish. They can supply acidity, bitterness, sweetness, aroma and the texture that keeps a rich sandwich from feeling heavy.
Tomatoes
Use ripe tomatoes for sandwiches eaten immediately. Slice thinly with a sharp knife and season shortly before assembly. For packed lunches, carry tomato separately or use roasted tomato, drained tomato relish or a less watery variety.
Lettuce and cabbage
Leaf lettuce creates broad layers but can slide. Shredded iceberg or cabbage distributes crunch more evenly and can hold a light dressing. Dry washed greens thoroughly; surface water is enough to soften bread.
Onions and scallions
Slice onions very thinly. A brief rinse can reduce harshness; a quick pickle adds acidity and color. Scallions and chives distribute onion flavor more gently in egg, tuna or chicken salad.
Pickles and peppers
Drain them before use. Chips fit flat stacks; spears suit rolls but can pull out. Pepperoncini, jalapeños, roasted peppers and olive salad each add moisture, salt and acidity, so reduce other sauces accordingly.
Roasted vegetables
Eggplant, zucchini, mushrooms, peppers and onions need browning and concentrated seasoning. Cool them for cold sandwiches and remove excess oil. Pair soft roasted vegetables with crisp cabbage, arugula, nuts or seeds.
Fruit in savory sandwiches
Apple, pear, cranberry relish and fig spread can work with sharp cheese or roasted meat. Use a small amount and add acid or bitter greens so the sandwich does not become dessert-like.