Buy a 3 lb Spaghetti Squash. Cut in half lengthwise and remove seeds. You will see the spaghetti sized bands of squash running crosswise along the shorter dimention. Do not remove the squash for they look similar. Simmer 2" of water in a large pot fitted with a steamer basket. Place squash on steamer basket, cut side down, without stacking squash. Steam for about 30 minutes or until squash is tender but not falling apart. Remove squash from heat and let cool cut side up until able to be handled, about 20 minutes.
Dice 1 Medium Onion and saute in 1 Tbl Butter til translucent, about 3 minutes. Scoop squash out of skin into a large bowl. Add cooked onion, 3/4 Cup Sour Cream, 3 Tbl Mayonnaise, 1/4 Cup Parmesam Cheese, 1/2 tsp Dried Basil, 1/2 tsp Salt, 1/4 tsp Freshly Ground Pepper. Stir until combined.
Butter a caserole dish appropriate for the volume of mixture. Dish can even be a souffle style dish, but don't fill all the way to the top. Add mixture and top with 1/2 Cup Crunchy Bread Crumbs (use Panko or Crushed Melba Crackers, for Gluten Free, use Gluten free Cereal or Cracker Crumbs), 1/4 Cup Parmesan Cheese and 1/2 tsp Paprika. Up to this point, can be made ahead. Refrigerate and set out for 30 minutes before baking.
Preheat oven to 350 and bake in hot oven for 40-45 minutes until bubbly. Delicious served with most all meats. Nice change for vegetarians too!