1 16-ounce package rigatoni
4 tablespoons butter, divided
1 tablespoon olive oil
1 onion, sliced
2 tablespoons flour
2 cups milk
1 cup Swiss cheese, shredded
2 5-ounce cans of tuna, drained
1/2 cup Parmesan cheese, grated
1 teaspoon salt
1/2 teaspoon ground black pepper
3 tablespoons fresh chives, chopped
Cook rigatoni according to package directions. Drain and set aside.
While pasta is cooking, heat 2 tablespoons of butter and 1 tablespoon of olive oil in a medium-sized skillet over low heat.
When butter has melted, add onion slices and toss to coat in fat. Cook onion slowly, stirring regularly, until the onions have begun to caramelize, about 25 to 30 minutes.
While the onions are cooking, melt the remaining 2 tablespoons of butter in a medium-sized saucepan over medium heat. When melted, whisk in flour until smooth. Gradually add the milk, whisking as you do.
Reduce heat to low and allow the milk mixture to cook for about 10 minutes until thickened. Remove from heat and stir in the Swiss cheese gradually.
Preheat the oven to 400 degrees F and grease a 9-by-13 casserole dish.
In a large bowl, mix the cooked rigatoni, cheese sauce, onions and tuna. Season with salt and pepper and mix thoroughly. Pour into the prepared casserole dish.
Place the casserole dish into the preheated oven and cook for 20 to 25 minutes, until the casserole is bubbling throughout and the cheese is golden.
Garnish with fresh chives before serving.