Place the chopped onions in a non-stick skillet over medium heat. Soften for 5-6 minutes. Meanwhile cut the beef into small bite-size pieces.
Pour the hot onions, beef, and all remaining ingredients, excluding the mashed potatoes and cheese, into a 6-quart slow cooker. Season with 1 teaspoon salt and 1/4 teaspoon pepper, and stir well. Wipe down the sides of the slow cooker with a dry paper towel. Then spread the mashed potatoes over the top of the beef mixture all the way to the edges of the crock to seal in the ingredients below.
Place a long piece of paper towel over the slow cooker to absorb the steam, and cover with the lid. Turn on high for 4-5 hours, or low for 8-10 hours.
When ready to serve, sprinkle the cheese over the top of the potatoes and place the lid back on for 5 minutes to melt the cheese. Serve hot and garnish with extra herbs if you have them.