Heat a large skillet over medium-low heat and add 1/2 tablespoons olive oil. Add in sweet potato, onion, 1/4 teaspoon of salt and 1/4 teaspoon of pepper, stir, cover and cook for 10-12 minutes, or until potato is soft. Remove lid and add garlic, cooking for 30 seconds.
Transfer potato mixture to a large bowl (slightly mashing potato with a fork) and add quinoa, breadcrumbs, cheese, herbs, remaining salt and pepper and mix well. Once combined, add in egg, then mix until moistened. Using your hands to bring it together, form four equally-sized patties. Heat the same skillet over medium heat and add olive oil. Add cakes and cook for 3-4 minutes per side, or until golden brown. Serve with blackberry salsa!