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*Note: You may want to bake your casserole on top of a baking sheet, as this does have a tendency to spill over in the oven.
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Preheat oven to 400 degrees F.
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For the potatoes:
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Place a large saucepot, filled with water, on high until water boils.
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Wash and cut potatoes into 1-inch cubes.
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Once water is boiling, place potatoes into the water.
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(Note: I always use a large spoon to drop potatoes into boiling water for safety!)
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Boil potatoes for 10 minutes or until fork tender.
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Drain potatoes in a colander and return potatoes to pot.
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Mash with a large fork or potato masher until large lumps are removed.
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Add milk, butter, cheese, garlic and salt and pepper to taste.
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With a handheld electric mixer, beat until fully incorporated and creamy, i.e.
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youll still notice some lumps but potatoes are mostly smooth.
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Taste and adjust salt and pepper if needed.
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For the filling:
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Heat a large skillet on medium heat.
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Add butter and turn skillet until completely melted.
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Add onion, carrots, mushrooms and celery and cook until veggies begin to sweat (about 7 minutes).
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Add beef and continue to break up with a wooden spoon.
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As beef begins to cook and release juices, add garlic to pan.
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Season with salt and pepper.
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Once beef is fully cooked, add flour and toss until fully incorporated.
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Immediately, add beef broth, tomato paste, Worcestershire, soy sauce and Dijon mustard.
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Stir well until evenly distributed.
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Add in frozen vegetables.
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Bring mixture to a boil to allow sauce to thicken.
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Add salt and pepper to taste.
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Once boiling, remove immediately to an 8 x 8 casserole dish that has been sprayed with cooking spray.
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Top with mashed potatoes.
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Bake for 25 30 minutes until mixture is bubbling and potatoes are golden brown.
