Make the meatballs: In a medium-sized mixing bowl, mix the ground beef and pork, onion, garlic, egg, salt, and pepper with your hands until just combined. Form the mixture into meatballs 1½ inches in diameter.
Melt the ghee in a large oven-safe skillet over medium-high heat. Add the meatballs and cook for 5 minutes, turning frequently, until browned on all sides. Transfer the skillet to the oven and bake for 18 to 20 minutes, until the meatballs are cooked through.
While the meatballs are cooking, make the sauce: Place all of the sauce ingredients in a medium saucepan over medium-high heat and whisk to combine. Bring just to a boil, then reduce the heat to low and simmer for 3 to 5 minutes, until the sauce has thickened, stirring occasionally. Taste and add additional chipotle powder or dry mustard if desired.
Once the meatballs are done cooking, use a slotted spoon to transfer them to the pan with the sauce and toss to coat. Sprinkle with chopped green onion, if desired, and serve.