In a medium skillet, heat 2 tbsp. oil over medium. Add the onion and leek and a large pinch of salt. Cook, stirring often, until the vegetables start to soften, 3 to 5 minutes. Add the tomato, carrot and garlic and a few more pinches of salt. Cook, stirring often, until the vegetables are soft and most of the tomato juices evaporate, 8 to 10 minutes. Let the vegetables cool.
In a large bowl, combine the chuck, veal, Parmesan and breadcrumbs; season with salt and pepper. Using two forks, mix until just combined. Add the vegetables and egg. Using your hands, gently mix just to blend. Shape into about thirty 1 1/2-inch meatballs.
Add a thin layer of oil to a large nonstick skillet; heat over medium. Working in two batches and adding more oil in between, cook the meatballs, turning often, until browned all over, about 4 minutes. Cover; reduce the heat to low. Cook until the meatballs are cooked through, 5 to 8 minutes. Transfer to paper towels to drain. Toss with the spaghetti and sauce.