Make the sauce: Heat the olive oil in a medium saucepan over medium heat. Add the onion, celery, carrot and a pinch of salt and cook, stirring occasionally, until soft, about 8 minutes. Stir in the tomatoes and their juices, the bay leaf, basil and 1/2 cup water. Bring to a simmer and cook, stirring occasionally, until the sauce thickens, 30 to 40 minutes. Season with salt.
Meanwhile, make the meatballs: Put the potato in a small saucepan with cold water to cover; add 2 teaspoons salt and bring to a boil. Cook until the potato is tender, about 10 minutes. Cool slightly, then coarsely grate the potato with a box grater.
Mix the ground veal, parmesan, milk, garlic, thyme, 1 tablespoon olive oil, 1/4 teaspoon pepper and 1 teaspoon salt with your hands until just combined. Gently form the mixture into 1-inch meatballs.
Heat the remaining 3 tablespoons olive oil in a large nonstick skillet over medium-high heat. Add the meatballs and cook, turning occasionally, until browned and just cooked through, about 8 minutes. Transfer to the sauce and simmer 5 minutes.
Fill the baguette pieces with the meatballs and top with more sauce. Sprinkle with parmesan.