Scrub and rinse the beets well, then place them into a medium saucepan. Fill the saucepan with water, making sure it covers the beets by about an inch. Place the saucepan over high heat and bring the water to a boil, then lower the heat and let the beets simmer until they're easily pierced with a fork, which can take anywhere from 35 minutes for small beets to over an hour for very large beets. Add hot water to the pot if it reduces too much.
Drain the water when the beets are done boiling, then briefly rinse them with cold water. Let them sit until they're cool enough to handle. Be careful here, as the outside can sometimes feel cool while the inside is still very hot.
Once the beets are cool enough to handle, use your hands to remove the peel by gently rubbing them. It should slip off easily.
Quarter then slice the beets and place them into a medium bowl. Add the balsamic vinegar, red wine vinegar, sugar, pepper, and salt, then stir to evenly coat the beets. Let them sit while you prepare the rest of the ingredients.
To make the dressing, add all of the ingredients to a jar or small container with a lid, then shake until the dressing is emulsified.
To make the maple walnuts, first preheat the oven to 400°F.
Place the walnuts, maple syrup, and salt into a small bowl, then stir until the walnuts are evenly coated.
Arrange the walnuts in an even layer on a baking sheet. Bake the walnuts for about 5 minutes, until most of the syrup has dried up and the nuts are lightly browned. Keep a close eye on them to prevent burning.
Let the walnuts cool on the baking sheet while you assemble the salad. They will crisp up as they cool.
Optional step: Place the onion slices into a small bowl and cover them with cold water. Let them soak for 5 to 10 minutes, then drain and rinse them. This will reduce the raw onion bite.
To assemble the salad, place the greens into a large salad bowl. Add the marinated beets, red onion, avocado, and maple walnuts. Drizzle with as much dressing as you like, gently toss, and season with salt and pepper to taste, if desired.
Serve.