Cook pasta according to package directions. Melt the butter over medium-high heat. Saute the onion until it's translucent. Add the flour and cook for one minute. Turn off the heat and add the milk gradually, while whisking the entire time. Once all the milk is incorporated, return the pot to the heat and cook until thickened. Gradually add the cheese, stirring until it's all melted. Add the tuna, peas and drained pasta. Pour the mixture into a greased glass casserole dish. Top with breadcrumbs. Bake at 350F for 20-30 minutes. Yield: 6-8 servings.