In a large bowl, mix the breadcrumbs and milk together and let sit for 5 minutes.

Add the remaining meatball ingredients to the bowl and mix gently until combined.

With your hands, roll ~1 1/2" meatballs (makes roughly 15-18) and set them on a parchment paper lined baking sheet.

Heat a large pan or pot to medium heat and add the olive oil, onions, water, and salt, and mix to coat. Cover with a lid and cook for 15 minutes.

Remove the lid and continue to cook, stirring occasionally. Adjust the heat to avoid burning and add a splash of water if necessary.

Heat a large oven-safe frying pan to medium heat with olive oil to coat the whole pan. Once hot, fry the meatballs until browned (about 3 minutes per side) then remove with a slotted spoon to a dish and set aside.

Preheat oven to 450°F and set the rack on the middle level.

Pour off the oil used to fry the meatballs and add all of the onions to the pan along with the butter. Cook the onions over medium heat until deeply caramelized, stirring frequently and adding water as necessary.

Once the onions are done, add the garlic and cook for 1 minute. Add the flour to the pan and cook for 1-2 minutes, stirring frequently, until the flour is no longer white.

Add the wine and bring to a boil, scraping the bottom of the pan with a wooden spoon to dislodge brown bits.

Once boiling, add the chicken stock and thyme and bring to a simmer for 5 minutes. Taste and adjust salt and pepper. Add the brandy or sherry (optional) and stir to combine.

Add all of the meatballs to the pan and top with 3/4 of the Swiss cheese. Place in the oven and bake for 7-10 minutes or until bubbly.

Top the baguette slices with the remaining cheese and place under the broiler for a minute to toast and melt the cheese, watching closely to prevent burning.

For a browner top, broil the meatballs for 2-3 minutes but watch carefully. Garnish with thyme leaves and serve with toasted baguette slices topped with the remaining Swiss cheese.