This first-course salad is best in the spring months, when the vivid, maroon-colored blood oranges, now grown commercially in California, make their appearance. Blood oranges have a distinctively rich orange flavor, with raspberry overtones, that has been featured in the Mediterranean diet for centuries. This update on the classic grapefruit and avocado combination includes thinly sliced white mushrooms, which offer a pleasant textural contrast. You can serve this as a first-course salad or make it into a main-course salad with shredded cooked chicken breast, fresh crabmeat, or shrimp. Serve a baguette on the side with herbed butter. Blood Orange, Mushroom, and Avocado Salad Serves 4 to 6 For the Dressing 1 medium shallot, finely chopped Juice of 1 medium blood orange (about 1/3 cup) 1 tablespoon balsamic vinegar 1/2 cup olive oil 1/2 teaspoon salt 1/4 teaspoon black pepper For the Salad 2 medium heads of butter lettuce, torn into …