3 tablespoons fresh lemon juice
2 tablespoons extra-virgin olive oil
1 tablespoon balsamic vinegar
1 tablespoon Dijon mustard, or more to taste
salt and ground black pepper to taste
1 pinch garlic salt, or more to taste
2 medium whole (2-3/5" dia) tomatoes, cut into bite-size pieces
½ red onion, thinly sliced
1 avocado - peeled, pitted, and cut into bite-size pieces
1 (4 ounce) container crumbled feta cheese
1 (8 ounce) package baby spinach leaves
Place beets into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender when pierced with a fork, about 1 hour. Drain and cool until beets can be easily handled. Peel beets, cut into cubes, and place in a bowl; refrigerate until chilled, about 15 minutes. Whisk lemon juice, olive oil, balsamic vinegar, mustard, salt, pepper, and garlic salt together in a large bowl until smooth. Add beets, tomatoes, onion, avocado, and feta cheese; toss. Top salad with spinach and toss.