As I can see, this works almost like a callback function system.
Signals and slots are similar to that, yes. But in order for your dialog to do anything useful, it has to be able to communicate that to the rest of the program. So the callback (MainWindow slot in this case) has to receive data from the dialog. When calling this slot (by way of a signal), the dialog has to pass that information along.

So an alternative to my pattern above would be similar to what is used in QFileDialog: whenever the user changes to a new directory, selects a different file extension from the combobox, or selects a file, QFileDialog emits a signal that notifies of the change. For your AddRecipe dialog to work the same way, it would have to emit a signal that contains the data the user has entered for the new recipe, and MainWindow needs a slot to receive that data. A signal / slot pair that don't transmit any information (like your show_table() slot) means that you have to find some alternative means to get the information from the dialog.

I use the QFileDialog pattern when I need to keep track of changes occurring in the dialog while the dialog is still posted (like an "Apply" button that updates the main UI based on the current settings being defined in the dialog). If I don't need that, I generally use the pattern I described in the previous post.