Preheat airfryer or oven to 375°F (190°C) and prepare flax egg in a small dish.
In the same skillet that the tempeh was browning, heat 1 Tbsp, (15 ml) of oil over medium-high heat. Add the onions and garlic and sweat them out for about 3 minutes, until translucent and fragrant. Then add mushrooms and a pinch of sea salt. When the mushrooms start releasing water, add the dried oregano, basil, and black pepper. Keep sauteing until all the liquid from mushrooms evaporates.
Store leftover meatballs with sauce covered in the refrigerator for a few days, though best when fresh.
*You can make flax egg by mixing 1 Tbsp. (7 g) flaxseed meal with 2.5 Tbsp. (37.5 ml) water.