Heat the butter in a skillet over medium heat. Add the shallot and garlic and sauté until they soften, about 2-3 minutes.
Add the tomato paste and cook for 1-2 minutes.
Sprinkle in the flour and mix it to form a roux, stirring constantly to prevent burning.
Slowly stir in the sherry, stirring/whisking constantly.
Slowly stir in the milk, adding a little at a time, stirring constantly, until a thick liquid/paste forms.
Add the seasonings (salt, pepper, paprika, tarragon) and cook for 1-2 minutes, stirring.
Slowly pour in the seafood stock while stirring/whisking constantly to prevent it from breaking.
Stir in the lobster chunks and cook for 30-60 seconds so the lobster meat cooks.
Remove from heat, stir in Parmesan cheese (optional) and serve over lobster ravioli.
How to Blanch Lobster Tails:
Heat a pot of water to boiling. Place the lobster tails in the boiling water for 1 minute.
Remove from the water and let cool. Using kitchen shears, cut the underside of the shell and gently peel it back to reveal the lobster meat.
Slowly remove the lobster meat from the shell. Chop into 1" chunks to add to the sauce.
Notes
Substitute white wine for the sherry if needed (I recommend the sherry if you can though).
You can use store-bought lobster ravioli or any other type of store-bought ravioli or pasta to serve with this sauce.
Stir/whisk the sauce continuously when adding the sherry and milk to ensure it doesn't clump.
If the sauce is too thick stir in more seafood stock until it reaches the desired consistency. If too thin, stir in some reserve pasta water until it is thin enough.
The fresh tarragon, lemon and Parmesan cheese are options, but I highly recommend using them for the best results.
You can omit the lobster to keep the recipe more frugal if needed.
Frozen lobster ravioli can be used as well - just follow the package instructions.
Adjust the salt to taste - different stocks have different levels of sodium so it is important that you are tasting the sauce while cooking.