Heat one tablespoon of olive oil in large saute pan on medium heat. Add the chopped onions and fennel and cook for 2-3 minutes, or until slightly translucent. Remove from the heat to cool.
In a large mixing bowl add the pork, eggs, parsley, fennel seed, salt and pepper. Once the chopped onions and fennel have cooled slightly add them to the mixing bowl and use your hands to mix all ingredients together.
Shape the pork mixture into small meatballs, approximately 1.5 inches in diameter. Make all the meatballs and place on a sheet of parchment paper.
Heat your previously used large saute pan with 3-4 tablespoons of olive oil on medium heat. Place the first batch of meatballs in the pan, being careful not to crowd the pan. Cook the meatballs 2-3 minutes on one side, then rotate them, so that all sides are browned. The meatballs should cook for 15 minutes total or until the inside is no longer pink. When the first batch is done, remove to a paper towel and start the next batch.
As the meatballs are cooking heat a separate saute pan on medium heat with 1-2 tablespoons of olive oil. Add the sliced fennel and cook for 4-5 minutes or until softened. Add the garlic and cook another minute. Lastly, add the spinach and stir together until the spinach begins to wilt.
Serve immediately by plating a portion of the spinach with 3-4 meatballs.