Preheat oven to 400 degrees.
Heat olive oil in a medium sauté pan over medium-high heat. Add onions, garlic and fennel seeds and sauté until the onions are soft and translucent, approximately 8-10 minutes. Add tomatoes and reduce heat, simmering for 20 minutes, stirring occasionally. Transfer to a blender and puree. Return sauce to pan, season to taste with salt and add basil and meatballs. Simmer until meatballs are heated through, approximately 8 minutes.