Tkinter Recalling A Function After Button Click

I'm using the answer from this Switch between two frames in tkinter to create my GUI.

For the first frame StartPage I'm trying to display the data from database using a function called DbConfig().fetch_data().

The second frame NewStudentPage is used to insert new data.

I'm trying to figure out how to recall the DbConfig().fetch_data() in StartPage if that frame is visible or after I click the button save (insert and redirects) from the NewStudentPage so I could display the data along with the newly inserted.

class SampleApp(Tk):
  def __init__(self, *args, **kwargs):
    Tk.__init__(self, *args, **kwargs)

    container = Frame(self)
    container.pack(side="top", fill="both", expand=True)
    container.grid_rowconfigure(0, weight=1)
    container.grid_columnconfigure(0, weight=1)

    self.frames = {}

    for f in (StartPage, NewStudentPage):
      page_name = f.__name__
      frame = f(parent=container, controller=self)
      self.frames[page_name] = frame

      frame.grid(row=0, column=0, sticky="nsew")

    self.show_frames("StartPage")

  def show_frames(self, page_name):
    frame = self.frames[page_name]
    frame.tkraise()

class StartPage(Frame):
  def __init__(self, parent, controller):
    Frame.__init__(self, parent)

    self.controller = controller
    Label(self, text="StandBy").pack(side="top", fill="x", pady=10)
    
    data = DbConfig().fetch_data()  <<<<<<--------------------- THIS FUNCTION
    print(data)

    Button(self, text="add student", command=lambda: controller.show_frames("NewStudentPage")).pack()


class NewStudentPage(Frame):
  def __init__(self, parent, controller):
    Frame.__init__(self, parent)
    self.controller = controller

    Label(self, text="New Student", font=Font(size="12", weight="bold")).place(relx=0.5, rely=0.1, anchor=CENTER)

    self.course = StringVar()
    Label(self, text="course").place(x=40, y=40)
    Entry(self, textvariable=self.course, width=25).place(x=80, y=40)
    
    self.firstname = StringVar()
    Label(self, text="firstname").place(x=15, y=65)
    Entry(self, textvariable=self.firstname, width=15).place(x=50, y=65)

    self.lastname = StringVar()
    Label(self, text="lastname").place(x=160, y=65)
    Entry(self, textvariable=self.lastname, width=15).place(x=190, y=65)

    Button(self, text="back", command=lambda: controller.show_frames("StartPage")).place(x=110, y=120)
    Button(self, text="save", command=lambda: [self.save_data(), controller.show_frames("StartPage")]).place(x=150, y=120)
  
  def save_data(self):
    course = self.course.get()
    firstname = self.firstname.get()
    lastname = self.lastname.get()
    DbConfig().insert_student(course, firstname, lastname)

class DbConfig:
  def __init__(self):
    self.conn = sqlite3.connect("database/config.db")
    self.cursor = self.conn.cursor()
    self.create_table()

  def create_table(self):
    self.cursor.execute("""CREATE TABLE IF NOT EXISTS tbl_student (
          _id integer PRIMARY KEY,
          course text,
          firstname text,
          lastname text
        )""")

  def insert_student(self, course, firstname, lastname):
    self.cursor.execute("""INSERT INTO tbl_student
      (course, firstname, lastname)
      VALUES (?, ?, ?) """, (course, firstname, lastname))
    self.conn.commit()

  def fetch_data(self):
    data = self.cursor.execute("SELECT * FROM tbl_student").fetchall()
    return data


if __name__ == "__main__":
  app = SampleApp()
  app.mainloop()