Peel the onions and cut them in half from root to stem. Slice thinly. Don’t worry about them being perfect or totally uniform.
Melt the butter in a large saucepan over medium-low heat.
Add the onions (they will likely fill the whole pot) and cook, stirring frequently, for 45 minutes to 1 hour, until they’re a dark golden brown.
While the onions are cooking, preheat the oven to 400°F (205°C).
Arrange the baguette slices on a baking sheet and toast them on the top rack of the oven for 3 to 5 minutes on each side, or until they’re nicely browned.
Add the water to the caramelized onions and use a wooden spoon to scrape the sides and bottom of the saucepan.
Add the salt and pepper and bring to a simmer. Cover the saucepan and simmer for 15 minutes.
Pour in the milk and return to a simmer over medium heat. Remove from the heat immediately once it starts to boil.
Ladle into bowls and serve it forth with toasted baguette on the side.
