Fill a large pot with water and heat on high. Bring it to a boil and when its boiling, add the pasta and cook for 8 - 10 mins. Drain, add it back to the pot and set aside.
Meanwhile heat a medium pot to medium heat. Add the oil and the onion and saute for a few mins. Add the minced garlic and cook for a min.
Next make a roux, with the butter and flour. Wait until the butter is melted before adding the flour. Cook for a couple mins while stirring until the butter and flour form a thick paste. You need to cook the roux for a little bit to get rid of the floury taste. You don't want your cheese sauce to taste floury.
Add the milk to the roux a little bit at a time stirring as you add it in. Keep stirring for a min or two until the mixture starts to thicken.
At this point, turn on your ovens broiler to heat up.
Remove the pot from the stove and add the cheese (save a little for the top) and stir till its all melted and combined. Add the dungeness crab and give it another stir.
Pour the cheese sauce over the pasta and stir to combine it well. Pour the pasta into a large casserole dish or a number of individual ones.
Make the topping by mixing the melted butter with the panko and salt (only use salt if you panko is not seasoned already)
Top the pasta with a little cheese and the panko topping. Throw it in the oven and broil until the cheese on top is browned and bubbling. Serve while its nice and hot!
Notes
This recipe makes a big casserole dish full or about 6 - 8 individual size servings.