Preheat the oven to 325°F.
Bring a large pot of water to a rolling boil. Salt it generously until it tastes like the ocean. Add the pasta and cook until al dente, about 7 minutes. Drain.
In a large bowl, whisk together the eggs, seasoned salt, and black pepper. Add the evaporated milk and whisk until fully combined. Set aside.
Return the drained pasta to the hot pot. While it is still hot, stir in the butter until fully melted.
Add the cubed cheddar to the pot and stir until it starts to melt. Add the shredded mozzarella or gruyere and stir again.
Pour the egg and milk mixture into the pot and stir until everything is well combined.
Pour the mixture into a 13x9-inch glass casserole dish and spread evenly. Bake for 15 minutes.
Remove from the oven and stir gently to redistribute the custard. Smooth it back into an even layer, then top with the shredded cheddar.
Return to the oven and bake for another 15 minutes, or until the cheese is fully melted and the eggs have set.
Optional: Switch to high broil for the last 5 minutes to toast the cheese and get crispy edges. Watch it closely.
Remove from the oven and let rest for at least 15 minutes before serving.