Salmagundi is more of a concept than a recipe. Essentially, it is a large composed salad that incorporates meat, seafood, cooked vegetables, raw vegetables, fruits, and nuts and is arranged in an elaborate way. Think of it as the British answer to Salad Niçoise.
Dating back to the seventeenth century, salmagundi refers to the large assortment of ingredients that give the diner a large range of flavors and textures on one plate. But it's also, in true British form, a way to use leftover roasts and vegetables in a new, striking way. The meal-sized salad is normally served for lunch or as part of a picnic.
While I have included recipes for an assortment of ingredients to serve in your salmagundi, don't feel inhibited. Use what is fresh, or what you have in the fridge, or what you feel like eating at that moment. Just make sure to include a wide variety of ingredients and plate the dish on a large serving platter, for the best first impression when you place your salmagundi on the table.