2 10-ounce packages frozen chopped spinach, thawed and squeezed dry
3 6-ounce cans chunk white albacore tuna, drained and flaked
2 cups small-curd cottage cheese
2 cups shredded Swiss cheese
4 large eggs, well beaten
Cook spaghetti according to package directions just until tender to bite. Drain well. Melt 1/4 cup of the butter in a wide frying pan; pour into a large bowl and mix in spaghetti and Parmesan cheese. Spread mixture in a greased 9-inch baking dish; set aside.
Melt remaining 2 tablespoons butter in pan over medium heat. Add onion and cook, stirring often, until soft but not browned (about 10 minutes). Stir in dell weed, garlic salt, and mustard; then remove from heat and mix in spinach, tuna, cottage cheese, and 1 cup of the Swiss cheese. Pour eggs evenly over spaghetti, then spread with tuna mixture. (At this point, you may cover and refrigerate for up to 6 hours.)
Bake, loosely covered, in 350º oven until casserole is hot throughout and eggs are set (40 to 45 minutes). Uncover, sprinkle with remaining 1 cup Swiss cheese, and continue to bake until cheese is melted (about 5 more minutes). Let stand for about 5 minutes, then cut into squares.