Yeah, for crustaceans steam until just done, shell the meat and cool it, storing it in the fridge. Then make a stock from scratch using the steaming water and the shells adding the mirepoix and herbs. Once the stock is ready and strained, build your dish. I’d add the mussels and clams if using here.
At least that’s my take on this.
And the ‘butter’ is called ‘mustard’ around here and the best part.
The culinary school used to make a lobster cream sauce by slow simmering the shells (meat removed) in cream then straining. The head of the school once stood over me telling me to push harder on the ladle in the chinoise to get every drop of the lobster cream out. I think I pushed for 10 more minutes getting a whole extra drop before he nodded enough.