To make the oregano-garlic vinaigrette: In a bowl, whisk together all of the ingredients, including salt and pepper to taste.
To make the mint vinaigrette: Begin with a flavor infusion, combining 2 tablespoons of the lemon juice and the 2 tablespoons chopped mint in a small saucepan or sauté pan. Bring to a boil and remove from the heat. Let steep for about 10 minutes, then strain through a fine-meshed sieve into a small bowl. There will be 2 tablespoons in all. Add the remaining ingredients, including the 1/4 cup chopped mint and the 1 tablespoon lemon juice, and whisk together.
If using beans, toss them in 2 tablespoons of the vinaigrette and leave to marinate for about 30 minutes. In a small bowl, marinate the onion in 1 tablespoon of the vinaigrette for about 15 minutes to soften it and lessen its bite.
If the spinach leaves are large, remove the stems and tear the leaves into smaller pieces. If using a large beet, cut it in half and then into 1/4-inch slices. Cut small beets into quarters or eighths. In a large salad bowl, combine the spinach, marinated onions, beets and the beans and shrimp, if using, with the remaining vinaigrette. Toss well and sprinkle with the olives and feta cheese.