Mash the avocado with a fork. Add lemon juice and garlic.
Add yogurt and mix to combine. Add salt and pepper and mix to combine.
Set aside until ready to use.
In a large skillet, heat about 2 tablespoons of olive oil over medium heat. Cook sweet potato burger patties for 3-4 minutes on each side or until golden. (You can also consider baking the patties in the 375 (190C) preheated oven for 30-35 minutes).
While the patties are cooking, toast the burger bun halves, in the same skillet.
Add cheese on top of the patties about 2 minutes before the patties are finished cooking.
Assemble the burger.
Spread avocado yogurt sauce over the burger bun halves. Top with lettuce, add the burger with cheese, BBQ sauce if used, tomato and red onion slices. Top with the other half of the burger bun, secure with a skewer and enjoy!