Have the cooked pasta drained and set aside. Pan spray a large oven-safe casserole dish.
In a large sauté pan, begin by melting the butter over medium high heat, then add the celery and onion. Stir and cook the vegetables until they become translucent and begin to brown.
Next add the cream of mushroom soup and two cups of milk. Stir to incorporate.
Next add the tuna, salt, Ranch Seasoning, peas and sour cream. Stir again and bring to a simmer. Add the cooked penne and stir again to mix the ingredients. Remove from heat as soon as the ingredients are combined to avoid over-cooking the peas or over-reducing the sauce. Add additional milk if the sauce appears too thick or scant. Pour into casserole dish, and smooth out the top.
Sprinkle the parsley, croutons and cheese on top. Pop the pan in the oven and broil for 3 minutes until the cheese is melted and bubbly.
Serve and eat while hot.