Heat a small skillet over medium-high heat. Toast almonds in hot skillet until lightly browned and fragrant, about 5 minutes.
Pour lemon juice into a large glass bowl. Drizzle olive oil into the lemon juice while vigorously whisking to make a dressing; add chopped avocado and stir to coat. Gently stir spinach and orange segments with the dressing and avocado; top with feta cheese.