Add pasta to boiling water and cook for 10 min. Drain.
Meanwhile, in a skillet, saute onion, mushrooms, green pepper and garlic in butter till crisp-tender. Remove from heat. Drain pasta; add to veggie mixture.
In bowl, whisk together sour cream and basil. Add crab and sour cream/basil mixture to pasta and veggies. Mix well.
Transfer to 9x13 pan or 2-8x8 pans that have been sprayed with nonstick spray. Sprinkle with cheese.
To cook and serve now: Cover and bake at 350°F for 20 minutes. Uncover and bake 5-10 minutes longer till thoroughly heated.
To freeze and serve later; Cover with waxed paper or cling wrap, wrap tightly in foil or seal in ziptop bag, removing as much air as possible. Label and freeze. To serve; Thaw in refrigerator overnight. Heat as above.