In a bowl, combine flour and salt, then add milk and cream; mix until dough forms.
Knead dough briefly until smooth, wrap, and let rest for 15 to 20 minutes.
Cook the sliced onions in a pan with oil or butter over medium heat until soft and lightly caramelized, about 15 to 20 minutes.
Add white wine to the onions and cook until it evaporates, then season with salt, pepper, and thyme.
Preheat the oven to 375°F (190°C).
Roll out the dough and place it in a tart pan, spreading the caramelized onions evenly over the base.
Top with slices of goat cheese, sprinkle with walnuts, and add pieces of dried duck breast.
Bake the tart for 30 to 35 minutes until the crust is golden and cheese is slightly melted.
Let the tart cool slightly before slicing, then serve warm or at room temperature.