1. Preheat the grill for high heat. Season both sides of chicken breast halves with steak seasoning.
2. Lightly oil the grill grate. Grill chicken 6 to 8 minutes per side, or until juices run clear. Remove from heat, cool, and cut into strips.
3. Meanwhile, place the rotini pasta in a large pot of lightly salted boiling water. Cook 8 to 10 minutes, until al dente. Drain, and rinse with cold water to cool.
4. In a large bowl, mix together the cheese, onion, lettuce, and tomatoes. Toss with the cooled chicken and pasta to serve.