Preheat the oven to 400°F and line a baking sheet with parchment paper.
Heat a drizzle of olive oil in a skillet over medium heat. Cook the onion for 5–6 minutes until softened, then add the garlic and spinach and cook until the spinach is wilted. Remove from heat and let cool slightly.
In a large bowl, combine the ground beef, egg, feta, dill, parsley, mint, salt, and pepper. Add the cooled spinach mixture and gently mix until just combined.
Roll the mixture into 1½-inch meatballs. Heat 1 tablespoon olive oil in a skillet over medium-high heat and sear the meatballs in batches for 1–2 minutes per side, until browned. Transfer to the baking sheet.
Bake for 10–15 minutes, or until the meatballs are cooked through and reach an internal temperature of 160°F. Serve warm.