Line a rimmed baking sheet with parchment paper or foil for easier cleanup, and preheat oven to 350°F.
Set a medium skillet over medium heat. Add 1 tsp oil and chopped onion and sauté, stirring occasionally, until softened and golden, about 5–7 minutes. Transfer to a plate to cool.
In a large bowl, add all of the meatloaf ingredients and mix just until well combined.
Add meat to the pan and shape it into a meatloaf about 8 inches long, 4 inches wide, and 3 inches tall.
Bake uncovered at 350°F for 40 minutes.
In a small bowl, add the sauce ingredients and stir to combine.
Spread the sauce over the meatloaf.
Return to the oven and bake an additional 20 minutes or until the internal temperature is 160°F on an instant-read thermometer.
Rest the meatloaf for 10–15 minutes before serving.