In a medium bowl, stir together the cream of mushroom soup and milk. Set aside. In another bowl, stir together the tuna, broccoli, 1/2 cup of Cheddar cheese and half of the fried onions. Stir in 3/4 cup of the soup.
Divide the tuna mixture evenly between the tortillas and roll up. Place rolls seam side down in a lightly greased 20x30cm baking dish. Pour the remaining soup over the rolls and spread evenly using the back of a spoon. It will be a thin layer. Sprinkle the rest of the fried onions over the soup, then the tomato and finally the remaining Cheddar cheese. Cover with lightly greased aluminium foil.
Bake for 30 minutes in the preheated oven. Remove the aluminium foil and bake for 5 more minutes to allow the cheese to melt and brown slightly.