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Heat oil in a sauté pan over medium heat. When oil becomes hot, add onion and sauté until onions become translucent.
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Add ground beef and season with salt and pepper (to taste). Cook until browned. Drain excess grease.
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Add green beans, peas and corn. Cook an additional two minutes.
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Stir in tomato paste and reduce heat to a slight simmer for about 5 minutes.
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Spread filling evenly in the bottom of a 9x13 casserole dish.
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Top with mashed potatoes and smooth evenly over beef mixture.
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Bake at 350-degrees for about 20 minutes.
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Top with cheese and bake another 5 minutes.