I'd stick with a pretty mild cheese and keep it in the background for something as delicate as crab. Since you are already wanting onion and pepper you can give it a deviled crab influence
Preheat oven to 375, butter a casserole dish, and cook about 12oz pasta to package directions
2 Tbls Butter
1/2 cup very finely chopped onion
1/2 of a finely chopped red pepper, preferably roasted and peeled first
saute onion and pepper in butter until tender
Remove to a bowl
Now make the cheese sauce. Use a large heavy bottom pot such as LaCruset as you will be adding in the pasta later.
3 tablespoons butter
1/4 c all-purpose flour
Stir over medium low until you have a nice blonde roux then slowly whisk in
1/3 c white wine
1-1/2 -2 cups half-and-half
1-2 tsp Dijon mustard (or 1 tsp Coleman's mustard powder)
2 teaspoons Worcestershire sauce
6 oz grated cheese. I'd probably go with Swiss and White Cheddar
Once that is smooth, stir in your onions and peppers, then season with salt, fresh white pepper, and fresh nutmeg
Now gently fold in crabmeat and cooked pasta
Add to a dish and top with
1 cup soft bread crumbs mixed with 1 tablespoon melted butter
Bake at 375 for 20-25 minutes or until topping is golden brown.
If you like chives or green onions, that would make a nice garnish
