Turn oven to Broil - high.
Add oil to a large skillet, on medium-high heat. When the oil is hot, add the onions and salt, stir well. Cook until the onions are well caramelized, about 15 mins, stir occasionally.
Clear the center of the pan, and melt the butter, when hot, stir the all-purpose seasoning into the butter, let bloom, about 30 seconds.
Add the demi-glaze and port to the butter and herbs, stir well, let cook for 30 seconds.
Add the cream cheese and sour cream. Stir everything, well. Let cook until cheese is melted and heated through, about 1 min. Stir occasionally.
Give it a taste and adjust seasonings as needed.
Top with the Gruyere cheese, and then the garnish items.
Put the skillet under the broiler for a minute or two until the cheese is melted and browned. Keep a close eye on it, because when the cheese starts to bubble it will brown quickly! Remove from oven.
Top with more of the garnish items --if desired.
Serve with the crostini and enjoy!