Preheat oven to 350°F (175°C).

Butter a 13 x 9 x 2-inch baking dish to prevent sticking.

Chop the onion and green pepper into small pieces.

Melt oleo (or butter) in a skillet over medium heat.

Sauté the chopped onion and green pepper in the melted oleo until softened.

In a large bowl, combine the cream of mushroom soup, cream of chicken soup, Ro-Tel chili tomatoes (undrained), and Pet milk.

Mix the soup mixture well until smooth and homogenous.

Crumble the taco-flavored chips into smaller pieces before opening the bag to minimize mess.

Evenly spread 1/3 of the crumbled chips on the bottom of the prepared baking dish.

Layer half of the diced, cooked chicken over the chips.

Pour half of the soup mixture evenly over the chicken.

Sprinkle half of the grated Cheddar cheese over the soup mixture.

Repeat layers: 1/3 chips, remaining chicken, remaining soup mixture, and remaining cheese.

Top with the remaining 1/3 of crumbled chips.

Bake in the preheated oven for 30 to 40 minutes, or until the casserole is heated through and bubbly and the cheese is melted and golden brown.

Let the casserole cool slightly before serving.