Start by peeling and finely chopping the onion. In a skillet, heat 1 tablespoon of olive oil over low heat. Add the chopped onion and sauté gently for about 5 minutes, or until it becomes soft and translucent, releasing its natural sweetness without browning.

Transfer the sautéed onion to a large mixing bowl. Add the minced meat, along with the crushed garlic, chopped parsley, breadcrumbs, and the small egg. Season the mixture generously with salt and pepper. Using clean hands or a sturdy spoon, mix until just combined—being careful not to overwork the meat to maintain tenderness.

Shape the mixture into approximately 20 meatballs, each about the size of a ping-pong ball. Roll them gently between your palms to create even spheres.

In the same pan used to cook the onion, heat the remaining tablespoon of olive oil over medium heat. Brown the meatballs in batches, cooking them for about 10 minutes until they develop a rich golden crust on the outside. Remove from the pan and set aside.

To prepare the Spanish sauce, melt the butter and olive oil together in a separate saucepan over medium-low heat. Once melted, sprinkle in the flour and stir continuously to form a smooth roux. Allow the roux to toast lightly for 2–3 minutes, just until it begins to turn a light golden color.

Slowly pour in the reduced beef broth in a steady stream while whisking constantly. This helps prevent lumps and ensures the sauce becomes silky and cohesive. Simmer for a few minutes until the sauce thickens to a smooth consistency. Taste and season with salt and black pepper as desired.

Return the browned meatballs to the sauce, gently stirring to coat them evenly. Let the meatballs simmer in the sauce for another 10 minutes, allowing them to absorb the savory flavors. Stir occasionally and taste once more for final seasoning before serving.