For the sour cream avocado sauce: Put the sour cream, salt, avocado flesh and lime juice in a food processor and puree. Set aside.
For the burger: Preheat the oven to 400 degrees F.
Prepare the quinoa following the package directions. Turn off the heat and allow to cool by removing the lid.
Toss the carrots, beets, squash, quartered onion, 2 tablespoons oil and 1 teaspoon of salt on a baking sheet. Spread evenly in one layer and roast for 1 hour. Set aside to cool for 20 minutes.
Placed the roasted veggies in a food processor with the thyme, garlic and a large pinch of salt. Process until chunky but not a mush (smalls chunks of veggies are okay—they help give the burger texture and more personality). Place in a large bowl.
Fold the quinoa and breadcrumbs into the chunky veggie mix until well incorporated. Taste and season if necessary. Form 5 patties from the mixture, oiling your hands to keep the mixture from sticking.
Heat the remaining tablespoon oil in a nonstick skillet on medium-high heat. Add the patties and cook until browned, 3 to 4 minutes per side.
Toast the buns. Spread the buns with the sour cream avocado sauce. To the bottom buns add the patties, tomato slices, sliced onion, pickles and alfalfa sprouts. Add the top buns to close the burgers.
Kobocha squash makes an excellent substitute for the butternut.
The patties can be prepared 2 days ahead and refrigerated, covered. Bring to room temperature for 30 minutes before cooking.