In a 12” cast iron or oven safe skillet, heat the olive oil over medium heat. Cook the onion, garlic, and fennel until softened, stirring occasionally, about 10 minutes. Transfer to a medium bowl and return the pan to the stove for later.
When the mixture has cooled enough to handle, add the remaining ingredients to the bowl. Mix together well with your hands, then form meatballs, about 2 tbsp in size. I like to pass/ toss the ball from palm to palm, smacking it a bit to pack it together.
Heat the oven to 350F and heat the reserved skillet over medium high heat. Brown the meatballs 1-2 minutes per side (I find it easiest to turn them with tongs), then transfer to the oven and bake 20 mins more. The meatballs should have an internal temperature of at least 160F.
Enjoy the meatballs over pasta or in a meatball sub!