Preheat the oven to 400°F. Wash the beets and place them into a Dutch oven (or other oven safe container with a lid, such as a ceramic or heat resistant glass cooking pot). Drizzle the beets with avocado oil and sprinkle them with salt and pepper. Put the lid on and place the pot on the middle rack of the oven for 60-90 minutes, or until the beets are tender. Start testing for tenderness after 60 minutes by carefully piercing one or several of the beets with a sharp knife; the larger the beets, the longer they will take to fully cook.
Cut the peeled beets into bite size cubes and, preferably, let them cool completely.
Place the cubed beets in a large mixing bowl, along with the baby spinach leaves and sliced onion.
Serve immediately, garnished with a handful of roasted pumpkin seeds if desired.