Prepare burgers as recipe directs. Keep patties refrigerated until ready to cook.
Peel, pit and coarsely chop 1 avocado. Place in a food processor and add lemon juice, parsley, basil, scallions, garlic and anchovy fillets, if desired. Pulse several times to chop. Add mayonnaise and yogurt. Process, stopping and scraping sides of bowl as needed, until smooth.
Grill or broil burgers as recipe directs. Peel, pit and slice remaining avocado lengthwise. Place burgers on bottoms of buns. Divide sprouts among burgers. Top with avocado slices and spoon sauce on top. Serve immediately with assorted toppings, if desired.