Heat oven to 425 degrees. Wrap beets tightly in foil and roast until tender, about 1 hour. Once cool, remove skin and slice.
Meanwhile, toss bread pieces with 2 tablespoons oil, season with salt and pepper, and toast until golden, about 6 minutes.
Whisk vinegar, orange juice, and remaining 2 tablespoons oil. Season with salt and pepper. Drizzle over beets, avocados, oranges, spinach, and croutons. Season with salt and pepper. Top with sunflower seeds.