Preheat oven to 375 degrees F (190 degrees C). Grease a 9x11-inch baking dish.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the rotini, and return to a boil. Cook uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 8 minutes. Drain well in a colander set in the sink.
Melt the margarine in a large skillet over medium heat, and pan-fry the chicken breasts until the meat is no longer pink inside and the outsides are golden brown, about 15 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Cut the chicken into bite-size strips, and set aside.
Cook the onion and green pepper in the same skillet until the onion is translucent, about 5 minutes. Sprinkle the vegetables with salt and black pepper, then stir in the chicken soup base and water until the soup base has dissolved. Place the cooked pasta into the skillet, and stir to combine with the vegetables; mix in the chicken strips and Swiss cheese until thoroughly combined. Spread the mixture into the prepared baking dish.
Bake in the preheated oven until the casserole is hot and the cheese has melted, about 15 minutes.
