In a large pot, cook pasta according to package directions to al dente stage. Drain.
Heat a saute' pan or skillet to medium-high. After the skillet is hot add 2 Tablespoons of oil. While oil is getting hot, salt and pepper chicken. Add chicken to the hot pan and cook for 3 to 4 minutes on medium-high heat. Turn and cook for another 3 to 4 minutes. Cook time will depend on how hot your pan is. You can cut chicken open to test for doneness. Remove from pan when they're cooked.
Cook Bechamel Sauce. Heat a large pot on medium. (You can use the same pot you boiled the pasta in.) Melt butter in a hot pan, but don't let the butter brown. Add flour and whisk. Cook flour for 2 minutes. Whisk in milk and continue whisking to remove any lumps. Cook until sauce thickens for 4 to 5 minutes. Add 1 cup cheese and stir until cheese is melted.
Combine pasta, chicken, artichokes, spinach (squeeze excess liquid out with a paper towel then pull it apart into small pieces), and pimientos in a large bowl.
Pour sauce over pasta mixture and gently stir. Pour into a 2-quart casserole dish that has been sprayed with non-stick spray.
Top mixture with remaining 1 cup cheese.
At this point, you can refrigerate for 4 to 6 hours and cook later.
When you're ready to cook, preheat the oven to 375°F. Cook uncovered 25 to 30 minutes until cheese is melted, bubbly, and browned.
Cover and refrigerate any leftovers up to 2 days.