Mix evaporated milk, Parmesan cheese, and onion salt together. Let stand to thicken.
Arranged canned biscuits close together in the center of an ungreased cookie sheet. With fingers, press biscuits together to close spaces. Then press into one big, thin biscuit 12 inches across. Bake near the center of oven 5 minutes. Take from oven, loosen with pancake turner, but leave on the pan.
Spread cheese mixture almost to the edge of biscuit, cover with catsup or chili sauce, then with tuna. Sprinkle with oregano. Return to oven for 7 minutes or until the crust is golden brown. Serve hot.
In place of tuna, try canned shrimp, sardines, anchovies or mushrooms, or 1/4 pound cooked sausage or hamburger, strips of salami, Thuringer, pepperoni or bologna