Butter an 8 x 8 square or equivalent baking pan.
Place the 2 tablespoons butter in a skillet over medium heat.
Cook the onions, stirring frequently, for 30-60 minutes until onions are caramelized (soft and have a rich yellow color).
Remove from heat and place the cornbread cubes in a mixing bowl.
Preheat the oven to 375 degrees Fahrenheit.
Add the caramelized onions and chopped sage leaves to the cornbread cubes, lightly stirring to mix together.
In a separate bowl, whisk together the vegetable stock, cream and egg.
Pour the wet mixture over the cornbread mixture, stirring to combine. If the mixture seems too dry, add a bit more cream.
Place in oven and bake for 30-40 minutes until the top is browned and crusty.