In a food processor, combine pork, crabmeat, ginger, garlic, green onions, cilantro, sake, salt and pepper. Pulse until all ingredients are nicely combined.
Cover a cookie sheet with parchment paper. Lay wonton wrappers in rows on the prepared cookie sheet, then scoop about 1 teaspoon of the pork and crab mixture onto the middle of each wrapper.
Fill a small bowl with water. Dip your fingers in the water and then pinch up the sides of each wonton wrapper, pressing to seal. The top of the shumai will be open, it will look like a little drawstring purse (as you can see in the photos.)
Place a steamer basket over a pan partially filled with water and spray with cooking spray (or line with cabbage leaves, if you have them.) Heat up the water until gently boiling and then place shumai in steamer basket. You will need to steam the shumai in a few batches, they will not all fit at one time (plus, you want some breathing room so you can easily get them out without them coming apart.) Steam each batch of shumai for 8 minutes, or until the pork and crab mixture is cooked through. Remove from basket and begin cooking the next batch.
Once all shumai is cooked, place in a warm spot while you make the béchamel. You can also easily microwave the shumai for a bit once the sauce is ready if they get too cold.
To begin making the béchamel, heat the milk and salt in a small saucepan until gently bubbling. In a medium saucepan, melt the butter, then add the flour. Whisk continuously for about 3 minutes to form a white roux. Remove from heat and, once the roux has stopped bubbling, add the hot milk mixture and immediately begin whisking vigorously.
Set the saucepan back over medium high heat and, continuing to whisk, add in the lemon juice and bonito flakes. Continue whisking until mixture comes to a boil. Cook for one minute and then immediately spoon over the shumai.