Mash the mince with a fork in a large bowl; add the garlic, onion, jalapeño, parsley and seasoning and mix well. To the mixture add the water, egg, oil, and breadcrumbs to bind together.
To test, make a small meatball with your hands and fry it up in an oiled frying pan over a medium heat until no longer pink in the centre; use this for your judgement regarding taste for further seasoning etc.
Once happy, roll the remaining mixture into desired sized balls and fry on medium-high heat for about 10 to 12 minutes until browned and no longer pink in the centre, turning regularly to ensure that they maintain their round shape.
For the marinara sauce:
Meanwhile, for the sauce just fry the onion until golden brown, add the tomato passata, diced tomatoes and all other ingredients except the mascarpone cheese and grated cheese.
Add the meatballs to the sauce and simmer over a medium heat for 8 minutes, stirring regularly. In the last few minutes of cooking stir through the mascarpone.
To serve, slice the demi-baguettes in half lengthways and lightly toast, cut side up, under a hot grill until golden. Remove from the heat and spoon the meatballs on top then sprinkle with grated cheese. Return to the grill until the cheese is melted, serve hot.