Preheat the broiler with a rack in the middle position. Heat 2 tablespoons olive oil in a small saute pan over medium heat. Add the shallot, a small pinch of salt and a splash of water and cook until softened, about 5 minutes. Turn off the heat and let cool slightly.
Pour the buttermilk over the stale bread in a large bowl and let sit at room temperature until the bread soaks up most of the buttermilk, about 5 minutes. Lightly squeeze the excess buttermilk from the bread; discard the buttermilk but keep the bread in the bowl. Add the egg, parmesan, parsley, cooled cooked shallots and 1 teaspoon salt to the bowl with the bread. Use your hands to stir the bread mixture together until it is well combined with no large pieces of bread. Add the ground beef and sausage and gently mix all the ingredients with your hands until just combined. (Do not overwork the mixture or the meatballs will be tough.) Using a 1-ounce ice cream scoop or 2 leveled tablespoons as a measurement, scoop and form 24 meatballs and transfer to a baking sheet.
Broil the meatballs until they are browned all over, 4 to 8 minutes, depending on your broiler. Lower the heat of the oven to 375 degrees F.
Meanwhile, pour the tomato sauce into a medium saucepan and bring to a low simmer over medium heat. Gently nestle the browned meatballs in the sauce, cover and cook until just cooked through, about 5 minutes. Turn off the heat but keep the meatballs covered to stay warm while you make the crostini.
Arrange the baguette slices on a baking sheet. Lightly brush the remaining 2 tablespoons olive oil on both sides of the bread slices. Bake until light golden brown around the edges and crispy throughout, 12 to 15 minutes, turning the bread slices halfway through the baking time.
To serve, smear each crostini with a bit of the warm tomato sauce, then top with a meatball and basil.