• Preheat the oven to 350 degrees. Prepare two 9x13 casserole dishes with nonstick cooking spray.
• Cook pasta according to package directions and drain well.
• In a large (12 inch) skillet over medium-low heat, melt the butter and add the onions, bell pepper, and celery. Cook the vegetables until tender, about 5 minutes. 
• Add the flour and seasonings to the skillet and stir until the flour coats everything, cooking for an additional 2 minutes.
• Gradually add in the milk, whisking until thick and bubbly. 
• Remove the skillet from heat. Stir in the mayonnaise and half of the cheddar cheese until melted.
• In a 30 quart bowl, combine the pasta, tuna, cheese sauce, and pimentos.  Divide the mixture between the prepared casserole dishes.
• Sprinkle the remaining cheese over the top.
• Bake the casseroles for 40 to 45 minutes or until the edges are bubbling. 
• Temporarily remove the casseroles from the oven and add a layer of french fried onions. Return to the oven and cook for an additional 5 to 10 minutes or until the onions are crispy.
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To make ahead and freeze as a freezer meal:

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• Prepare casseroles as directed but do not top with french fried onions.
• Cover tightly and store in the refrigerator for up to 3 days or freeze for up to 6 months.
• Allow to thaw in the refrigerator overnight prior to baking.
• Uncover and proceed according to directions.