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.