Begin by boiling a large pot of salted water. Add the cubed potatoes and cook for about 15-20 minutes, or until tender. Drain and return to the pot.
While the potatoes are cooking, preheat your oven to 400°F (200°C).
In a mixing bowl, combine the minced garlic, ½ cup of grated Parmesan cheese, salt, pepper, Italian seasoning, and olive oil to create a marinade for the chicken and steak.
Place the chicken breasts and steaks in a large bowl, coat them with the garlic Parmesan marinade, and let them sit for about 10 minutes while you prepare the potatoes.
Mash the drained potatoes with a potato masher, adding the milk and butter to make them creamy. Stir in the shredded cheddar cheese until well combined, then season with salt and pepper to taste.
Heat a skillet over medium-high heat. Add the marinated chicken breasts and cook for about 6-7 minutes on each side, or until they reach an internal temperature of 165°F (75°C). Remove from the skillet and set aside.
In the same skillet, add the marinated steaks and cook for about 4-5 minutes on each side, or until they reach your desired level of doneness (130°F for medium-rare, 145°F for medium).
In an oven-safe dish, place the cooked chicken on one side and the steaks on the other, and sprinkle the remaining ½ cup of Parmesan cheese on top. Transfer to the preheated oven and bake for 10 minutes, allowing the cheese to melt and form a crust.
Serve the garlic parm chicken and steak hot over a generous scoop of cheesy mashed potatoes, garnished with fresh parsley if desired.