Preheat the oven to 400°F.
Place the hashbrowns on a tray lined with parchment paper and bake for 7 to 9 minutes on each side until golden.
To a bowl, add the tuna, mayo, lemon juice, red onion, corn, parsley, black pepper, salt, jalapenos, and yellow mustard. Mix together well.
Take the hashbrowns out of the oven and add the tuna salad mix on top. Top with sliced tomato and then shredded cheddar cheese.
Set the oven to broil and pop in the oven for 2 to 4 minutes until the cheese is golden and bubbly on top. Watch it closely so nothing burns.
Take out of the oven, sprinkle chopped parsley or chives on top, and enjoy!