Every meatball has a melty cheese surprise inside!
Preheat the oven to 425°. Line a baking sheet with parchment paper.
In a medium bowl, mix the breadcrumbs and milk. Let stand until the crumbs soften, 3 to 5 minutes. Add the beef. Season with salt and pepper. Add the chile paste, garlic, and fennel seeds. Mix in the kale, Pecorino, and egg. Roll into balls about the size of a large walnut. Press 1 piece of provolone into each meatball, then enclose the cheese with the meat. Arrange on the baking sheet; spray with cooking spray or brush with oil. Roast the meatballs, turning once, until golden and cooked through, about 18 minutes.
Meanwhile, for the sauce, in a large skillet, heat the oil, two turns of the pan, over medium. Add the garlic and stir until aromatic, a minute or two. Stir in the tomato paste, then the vermouth and oregano. Stir in the tomatoes, passata, stock, and basil. Season with salt. Simmer the sauce over low heat while the meatballs finish cooking.
Add the meatballs to the sauce and turn to coat.