Prepare the vinaigrette: Place shallots, vinegar, citrus juice, and salt in a small bowl. Let shallots macerate for 30 minutes. Whisk in the olive oil. In a large salad bowl, toss together lettuces, citrus, beets, herbs, avocado, and the vinaigrette. Taste and adjust seasonings. Serve immediately.
TIP: To turn this into an entree top with scallops, chicken or fish.