Cut a wedge out of the baguette over the entire length, take it out and remove a little more dough so that there is enough space for filling later.
Heat 1.5 tablespoon butter in a pan and fry the diced salmon all over. Remove from the pan and sauté the baby spinach in the same fat.
In a second pan, heat the remaining butter and sauté the diced onions and garlic until translucent. Sprinkle in the flour and briefly toast while stirring. Gradually stir in the milk with a whisk so that there are no lumps. Season with salt and pepper. Turn off the stove and fold in the salmon and spinach. Pour the salmon mixture into the well of the baguette and sprinkle with the cheese. Bake in a preheated oven for 20-25 minutes until golden yellow.
Let cool down briefly, sprinkle with chopped parsley and cut into pieces.