Add all dressing ingredients to a small jar. Seal and shake well until combined.
Add grated carrots to a small bowl. Pour in half the dressing. Toss and scrunch well. Let sit 1–2 minutes.
Add avocado to a small bowl with a squeeze of lemon, a pinch of salt, pepper, and garlic powder. Mash with a fork until smooth.
Toast bread, then let it cool slightly. Spread a thin layer of mayo on both slices.
Spread mashed avocado evenly on both slices. On the bottom slice, layer cucumbers, tomatoes, and a light sprinkle of salt and pepper. Add beet slices, then the dressed carrots.
Add spinach to the carrot bowl with the remaining dressing. Toss well, then pile onto the sandwich.
Add top slice of bread, press gently, and slice in half. Enjoy right away!
Video
Notes
Storage: Serve right away. This sandwich gets soggy if it sits too long.