In a large bowl, combine all of the meatball ingredients and mix with your hands until evenly combined and well incorporated. Shape into balls, about the size of a golf ball and place on a baking sheet lined with parchment paper or foil.
Set a large skillet over medium-high heat with 3 tablespoons olive oil. Once hot, brown the meatballs in batches, for about 3 to 4 minutes per side. We're not looking to cook them through all the way, just brown them and get a good sear. Transfer to a plate or platter and continue browning the rest, set aside.
Either in the same skillet or a clean cast iron skillet (depending on how big or dirty the skillet gets) heat another tablespoon of olive oil. Once hot, add the onions, garlic, and jalapeño and cook until soft and translucent, about 5 minutes. Stir in the crushed and diced tomatoes and season with salt, pepper, oregano, and crushed chili flakes. Lower flame, cover and simmer for about 10 to 15 minutes or until slightly reduced by half. Nestle the meatballs into the sauce, making sure to fit them into the pan in a single layer. Top with shredded cheese and bake for about 15 to 20 minutes.
Remove from oven, sprinkle with feta and garnish with basil. Serve warm with the grilled bread slices. Enjoy!